I followed the guidelines at https://docs.kentico.com/display/K82/Using+ASP.NET+Web+API+with+Kentico to build a custom Web API controller. Also I enabled Basic HTTP Authentication in the Kentico Administrator UI in Settings -> Integration -> REST -> Authentication type and decorated the ApiController with the Authorize attribute to limit authorization to a specific user.
When I do an HTTP POST request to the ApiController, I always receive the error "Authorization has been denied for this request.". What am I missing?