Hello, interesting approach, Eilert!

There are a couple errors in the script though:
1. The parameter @convertResponseToBas64 is missing (it's not optional in the C# code).
2. There is a typo in the first query (missing s in the variable name):

FROM OPENJSON(@reponse_json)

I would also suggest implementing some exception handling in the C# library. If an exception occurs (for instance, invalid URL, authentication problem, invalid request, etc.) the function will raise an exception and it won't return any response as it should according to the HTTP protocol specification.

Other than that, really nice work!