Where is redeem code for public link for TestFlight?

The redeem code is sent by mail when you add an external or internal new tester in TestFlight.

When you open your app in App Store Connect, go to "My Apps" and select your app. Then go to the "TestFlight" section, complete the "Information" section, and then add external testers as needed and the build to test.

After this Apple sends emails to all testers with the invitation.

A good tutorial on TestFlight can be found on raywenderlich.com.

[tl;dr user is doing it wrong + Apple designed this flow poorly.]

Presumably what is happening is some users who haven't previously installed TestFlight (or who opened the link in an app other than Safari, which won't automatically open TestFlight for them) are clicking on the public link, which if opened on an iOS device shows: "Step 1 Get TestFlight".

(notice that on many devices some portion of "Step 2" will be covered up, such as the critical "Start Testing" button in the screenshot above.)

And then instead of returning to the webpage after installing TestFlight and scrolling down and clicking the "Start Testing" button under "Step 2 Join the Beta", some users are mistakenly staying in the TestFlight app and either tapping the "Redeem" button as it is the only interactable element on the mostly empty default TestFlight screen or reading the subtitle that says to "contact the developer for an invitation code."

(Note the big title text would be confusing to users who thought this was how they start testing and they would naturally think at this point something has gone wrong.)

Unfortunately this is a natural consequence of the way this workflow is designed, and I don't see a simple way around it besides educating your users. Certainly one workaround is to invite the user via email, so they will get a code they can enter into the "Redeem" alert text field, but this should not be necessary.

It may seem obvious to technically-savvy people, but one can clearly see some people not understanding that they need to switch back and forth like that. Until this design flaw is fixed, a certain percentage will make this mistake. I suggest sending Apple some feedback about this.

This issue is happening to me also. Some customers are saying they are asked to enter the redeem code while using the public link for downloading the app from TestFlight, so they are unable to download the app.

Workaround was to add the user's email id to the test group so it will send email invites with redeem code.

But I still do not understand why the public URL for downloading the app is asking for a redeem code? And what will be the redeem code for people using the public link? Another interesting thing is it's happening only to some users.

My rep is too low to comment, but something you can also do is ask your testers to do a "long press" on the link. This will allow them to open the link in either Safari or TestFlight. Opening in TestFlight will sometimes resolve this issue.