TestFlight

You ask the developer of the specific app you want to test, and they invite you. You can do this by contacting them through the contact details that every app developer provides on their app pages in the App Store.

What you don't do is put a post on these Developer Forums about it, because:

1. The developer of the app you want to test is likely not on here. (There are millions of developers around the world, and not every developer is on here.)
2. Even if they are here, your post is not specific enough, so even if they saw it they wouldn't know it's their app you want to test.
3. These are the Developer Forums, where developers of third-party apps for Apple's platforms ask each other for hints and tips on coding. Asking for TestFlight invitations is nothing to do with these forums.