How can I pay T-Mobile bill with Apple Pay to receive 3% cashback?

Tmobile is listed as one of the merchants for which applepay cardholders receive 3% cash back. I did not see a means to add it to my tmobile account online, and went in to my (updated) tmobile app to add it. No go. Its AI says I can buy a phone this way, but it "cannot help" with adding it as my autopay source. Has anyone been successful with this yet?

[Re-Titled by Moderator]