I understand that despite giving correct inputs in your request while calling API, you are still getting an error. If you provide the error code/description, it would clarify things and help resolve the underlying issue.
As part of my day job, I do API automation and build API Bots using Python and C#. I have a good grasp on forming API request (headers, body, authentication etc.), parsing API response and extracting data from it.
To integrate the API calling code with your .NET software, we can either write this code in C#, or write it as a Python script and then call it from your software as an executable. This we can decide by looking at your existing set up.
If these are the skill sets you were looking for, I will be happy to help and impress.
Thanks & Regards,
Kamalesh