Azure Data Factory - help needed to ingest data, that uses an API, into a SQL Server instance

Hi,
I need to ingest 3rd-party data, that uses an API, into my Azure SQL Server instance (ASSi) using Azure Data Factory (ADF).
I'm a report developer so this area is unfamiliar to me, although I have previously integrated external, on-premise SQL Server data into our ASSi using ADF so I do have some exposure to the tool (just not API connections).

The 3rd-party data belongs to a company named 'iLevel' (in case this is relevant). iLevel have provided some API documentation which is targeted for an experienced data engineer that understands API connections. This documentation has just a few sections before the connection focused details end. I'll list them below:
1) It mentions to download 'Postman Collection' and mentions no more than this. I've never heard of Postman Collection and I don't know why it's needed. Through limited exposure online, I don't understand its purpose, certainly not in my scenario.

2) It has the title 'Access to the API' and then lists four URLs which are the endpoints (I don't know which to use and will need to ask iLevel of this but, I guess, any will do for testing purposes).

3) Authentication and Authorization
a) Generate a 'client id' and 'client secret' by logging into iLevel and generating these values with some clicks of a button. I've successfully generated both these values.
b) Obtain an Access Token - it'll be easier to screenshot the instructions for this (I've blanked part of the URL for confidentiality).

These are all the instructions on connection to the 3rd-party data. Unfortuntely for me, my lack of experience in this area means these instrcutions don't help me. I don't believe I'm any closer to connecting to the 3rd-party source data.

Taking into consideration the above instructions but choosing to try and error in ADF, a tool I'm a little bit more familiar with, I've performed the following steps:
1) Created a Linked Service.
I understand the iLevel solution is in the cloud and therefore the 'AutoResolveIntegrationRuntime' option has been selected as the 'Connect via integration runtime' value. For the 'Base URL' I've entered one of the four URL endpoints that were listed in the documentation (again, I will need to confirm which endpoint to use).

The 'Test Connection' returns a successful result but I think it means nothing because if I were to placed 'xxx' at the end of the Base URL and test the connection, it stills returns successful when I know the URL with the 'xxx' post-fix isn't legit.

2) Create an ADF Pipeline containing 'Web activity' and 'Set variable' objects.
The only configuration under the Web activity is the 'Settings' pane which has:

The 'Body' property has (the client id and client secret taken from the iLevel solution are included in the body but blanked out):

If the Web activity is successful then the Pipeline's next object (the Set variable) should assign the access token to a variable - as I understand this is what the Web activity is doing:

The 'Value' property has:

This is as far as I've got into my efforts of this integration task because the Web activity object fails when executed. The error message does state it is to do with an invalid 'client id' or 'client secret' - see below:

You may direct to focus on the incorrect client id or client secret, however I don't have any confidence that I understand how to configure ADF to obtain an access token, and I'm maybe missing something if I see no need for Postman Collection use.

What is Postman Collection and do I need it for what I'm trying to achieve?  If yes, can anyone provide training material that suits my need?
Have I configured ADF correctly and it is indeed an issue with the client id or client server, or is the error message received just a by-product of an incorrect ADF configuration?

Your help will be most appreciated. Many thanks.