To test beta versions of iOS and AppleTV apps using TestFlight, you’ll need to accept an email or public link invitation from the developer and have a device that you can use to test. You’ll be able to access the builds that the developer makes available to you.
Installing a beta iOS or iPadOS app via email invitation
Install TestFlight on the iOS device that you’ll use for testing.
Open your email invitation or tap the public link on your device.
When installing via email invitation, tap “View in TestFlight” or “Start testing” then tap “Install” or “Update” for the app you want to test.
Installing a beta tvOS app via email invitation
Install TestFlight on Apple TV.
Open your email invitation on a mobile device or computer.
Tap or click Start Testing. You'll be taken to a web page with a redemption code.
Open TestFlight on Apple TV.
Go to Redeem and enter the redemption code.
Reference article: https://testflight.apple.com/