C# oAuth2 Class -- 2

I need a simple oAuth2 class for a merchant gateway API. The authorization endpoint utilizes Amazon Cognito. The endpoint requires *both* key/secret *and* username/password (basic) authentication. No matter what I try, I can't get the endpoint to accept my encoded username/pass. I can authenticate successfully with Postman, but not in C#. Obviously, the problem is with my code. This project is considered complete when you can successfully get the initial token and refresh token. I'll send the successful bidder the .Net Standard WinForms solution with all of the required connection info (and my failed attempt at authenticating). This opportunity is *only* available to freelancers from North America, South America, Europe, or South Africa. Budget is $30 - $50 as it shouldn't take more than an hour or two.