Missing query parameter 'responseUri'

Tomtit Like Здравко said, I also recommend starting with an unmodified sample app to see that working first, before making changes or implementing your own.

Anyway, as they pointed out, your code that you shared here is incorrect because it is passing the authorization URL itself to ProcessCodeFlowAsync. That won't work because the authorization URI is the location of the web page where the user needs to manually authorize the app once. It's the resulting redirect URI (containing the authorization code) that you should pass to the responseUri parameter of ProcessCodeFlowAsync, like I described in your other thread. (Alternatively, you can pass just the authorization code string as the 'code' parameter.)