SOLVED: After a bit more forum searching I found a relevant thread that solved the issue.
A question regarding API’s.
I am not a coder, so please forgive me if some terminology is not right.
I have successfully connected to makeplans.net’s API, after a couple of hours of trying and failing with authentication 
Is it possible (and if so, how?) to use “dynamic values” in the GET url?
The API documentation for the service I’m connecting to says this:
“GET /services/{service_id}/slots will return slots for specified service.”
The service_id part is what needs to be dynamic.