Setting DevOps Pipeline variable

I have created a DevOps Pipeline that is triggered when the developer checks in the code. the pipeline has a branch, dependent on the value of a Pipeline library variable.

I don't want to require the developer to have to change the variable in DevOps each time before they check in the code.  

I am looking for a way to control that from within Visual Studio Professional

I thought that i could create a JSON file that contains the variable and the developer would set the value in the file before checking in. I could have the first pipeline step set the variable and then the following steps could use the value. I have not found a task that would do that.

Could someone recommend a task to me? Or is there a better way to do this?

thanks