It would help to know what your goal is and and what service you are trying to call to AppSheet from.
It might also help to know what documentation you have looked at so we don't give you the same info.
I would suggest two things:
1) If you haven't digested it yet, review this article:
https://help.appsheet.com/en/articles/1979979-invoking-the-api
2) Create an API call within AppSheet itself. You can have an AppSheet app call the AppSheet API. This might help you understand the pieces you need to successfully construct an API call from another external service.
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