How do I file an official complaint with apple.

I have had 3 calls with tech support over a two week period and nobody can tell me how to fix my issue. The issue is my iPad Pro microphone will not work with facetime. It works with every other app. The link they give you as a “solution” does not solve the problem. I want to file an official complaint but tech support can’t tell me how to do that. A “senior advisor” could not tell me how to do it. So, how can I file an official complaint?