How do I get a TestFlight redeem code on iPhone?

You don't get a TestFlight code, unless an app developer offers one to you.

https://testflight.apple.com/

"To test beta versions of apps and App Clips 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."

Then you get to be an unpaid guinea pig ("beta tester") for whatever the developer wants you to test.

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