I would recommend to capture the raw HTTP request from using twurl using ‘-t’ parameter (trace?) and compare with the raw HTTP request (you might have to use Fiddler or something to listen on network calls if your code doesn’t log it), the URL formatted call you wrote looks fine to me as long as accounts/account_id actually being replaced by the real account_id properly etc.
You should also check if you can do the same call with twurl - if you can’t there could be a problem with the app’s permissions.
Finally there is a c# SDK that some folks were working on - it would be great to have collaborators sort of rally around a solution: Twitter Ads API Integration With C# Application
Thanks,
John