So it sounds like you successfully generated all the necessary tokens and keys to make the API call, and the problem you are facing is setting up the actual API URL Endpoint to pull down the data.
Using your example of GL data, you can reference the link below that shows you how to make the request URL:
On the right you will see an example of the Request URL:
Try entering the above URL in the API Endpoint URL field for the tool as a test. You will need to change the sections in red where after "company/" is your company ID number and the dates will be your start and end date for the query.