Hi Keenan (@Ninjak), this complex auto-increment can be achieved relatively easily using 2 lists in SharePoint and a flow in Power Automate. The result will look like this in the main list which I've called Crop Harvest:
You will need a second list (which I've called Increment) to hold the next value. In the result above I manually changed the value to show the difference based on the number, and I've manually changed it back to 9 for now but you will see that it will auto-increment in the flow. This second list will only ever have 1 item which is the next value to be used:
In the flow the trigger I've used is When an item is created in the SharePoint main list.
Next we get the 1 item from the Increment list, and the next action is to initialize a string variable which I've named varIncrementNumber:
The next action to add is a Compose control which I've named ComposeNext and you select Next value from the dynamic content box on the right. What you will see when you do this is that it wraps itself in an apply to each control. It does this because although there is only 1 item in the Increment list there could potentially be lots so it adds the apply to each. That's fine, it's expected and you don't try to delete it but just leave it there.
The next action is a condition. For this first condition we are only going to test if the output of the previous compose action is less than 10 (we'll do the others shortly):
If it IS less than 10 then in the If yes green channel we add an Append to string variable and in the value type T00000 immediately followed by the output of the ComposeNext selected from the dynamic content box on the right:
If the ComposeNext value isn't less than 10 then in the red If no channel you add another condition. In the left hand box select the Output from the ComposeNext from the dynamic content, select is greater than or equal to, and then in the right hand box type 10. Then add another row and do the same but make it less than 100.
Again, you'll get a green if yes channel and a red if no channel. In the green channel, as before, add an append to string variable action and this time in the value add T0000 and then the output of the ComposeNext:
In the red If no channel add another condition as before but make it greater than or equal to 100 and less than 1000.
You keep doing that - changing the number of leading zeros each time of course - until you get to the last condition which is if the output of the ComposeNext is greater than or equal to 10000 and less than 100000 then you will add an Append to string variable in both channels. In the green If yes channel it will be T0 plus the Output and in the red if no channel it will be just T plus the Output.
So we've got the right value with the T plus correct leading zeros in our variable now. The next action is still inside the apply to each but not inside any of the conditions. The action is a SharePoint Update item action and in the text column where you are storing the increment text value you select the string variable varIncrementNumber. You'll see I've also stored the actual numeric value number from the ComposeNext:
We're almost at the end now and the next action is to add a Compose and add 1 to the ComposeNext value. So for this we use an expression of add (outputs('ComposeNext'),1). Finally we use the output of that to update the Increment list:
Whatever value you have in the Increment list the flow will check that and apply the T and the correct number of zeros and then the increment value. It will take just 2 or 3 seconds to do this and to update the main list and the increment list.
That took far longer to write out and do the screenshots than it took to prepare the SharePoint lists and do the flow!! 
Do come back with questions about any of this.
Rob
Los Gallardos
Microsoft Power Automate Community Super User
@RobElliott I have tried to set this up for a while, tried 3 times and done everything exactly, but it does not work.
I dont get an error when I try to test the workflow, it just does not work....
One thing is however different. Every time I set up the composenext, I get an additional step, without being able to remowe it. I cant see if you have that on your end as well, since you cut of the image :(
@Ninjak I realise this post is over a year old but thought it is worth highlighting that there is now a Format Number action that should make this process a lot more simple:
Accepted Solutions
Hi Keenan (@Ninjak), this complex auto-increment can be achieved relatively easily using 2 lists in SharePoint and a flow in Power Automate. The result will look like this in the main list which I've called Crop Harvest:
You will need a second list (which I've called Increment) to hold the next value. In the result above I manually changed the value to show the difference based on the number, and I've manually changed it back to 9 for now but you will see that it will auto-increment in the flow. This second list will only ever have 1 item which is the next value to be used:
In the flow the trigger I've used is When an item is created in the SharePoint main list.
Next we get the 1 item from the Increment list, and the next action is to initialize a string variable which I've named varIncrementNumber:
The next action to add is a Compose control which I've named ComposeNext and you select Next value from the dynamic content box on the right. What you will see when you do this is that it wraps itself in an apply to each control. It does this because although there is only 1 item in the Increment list there could potentially be lots so it adds the apply to each. That's fine, it's expected and you don't try to delete it but just leave it there.
The next action is a condition. For this first condition we are only going to test if the output of the previous compose action is less than 10 (we'll do the others shortly):
If it IS less than 10 then in the If yes green channel we add an Append to string variable and in the value type T00000 immediately followed by the output of the ComposeNext selected from the dynamic content box on the right:
If the ComposeNext value isn't less than 10 then in the red If no channel you add another condition. In the left hand box select the Output from the ComposeNext from the dynamic content, select is greater than or equal to, and then in the right hand box type 10. Then add another row and do the same but make it less than 100.
Again, you'll get a green if yes channel and a red if no channel. In the green channel, as before, add an append to string variable action and this time in the value add T0000 and then the output of the ComposeNext:
In the red If no channel add another condition as before but make it greater than or equal to 100 and less than 1000.
You keep doing that - changing the number of leading zeros each time of course - until you get to the last condition which is if the output of the ComposeNext is greater than or equal to 10000 and less than 100000 then you will add an Append to string variable in both channels. In the green If yes channel it will be T0 plus the Output and in the red if no channel it will be just T plus the Output.
So we've got the right value with the T plus correct leading zeros in our variable now. The next action is still inside the apply to each but not inside any of the conditions. The action is a SharePoint Update item action and in the text column where you are storing the increment text value you select the string variable varIncrementNumber. You'll see I've also stored the actual numeric value number from the ComposeNext:
We're almost at the end now and the next action is to add a Compose and add 1 to the ComposeNext value. So for this we use an expression of add (outputs('ComposeNext'),1). Finally we use the output of that to update the Increment list:
Whatever value you have in the Increment list the flow will check that and apply the T and the correct number of zeros and then the increment value. It will take just 2 or 3 seconds to do this and to update the main list and the increment list.
That took far longer to write out and do the screenshots than it took to prepare the SharePoint lists and do the flow!! Do come back with questions about any of this.
Rob
Los Gallardos
Microsoft Power Automate Community Super User