Get subscription invoice PDF via API

Get subscription invoice PDF via API

This one is kind of duplicate, but please read in complete before marking or judging so.

I following the following post response:
https://help.zoho.com/portal/en/community/topic/get-invoice-pdf-via-api

This does not work in my case. I am pretty sure I am making some client code mistake but not sure what.

I tried the same from Postman client - it shows PDF to save but the PDF does not open.
Did the same from code C# but that gives me "Bad Request" on call.

Not sure how to diagnose the problem and make progress, need help @Raghav or any one from the team please.

Thanks in advance!