You must tap a link in the mobile app or on TuneIn’s website to log in with your Apple Music credentials and give TuneIn permission to stream Apple Music Radio. Or, access the Music section within the TuneIn settings to set this up.
The partnership includes calls to action in the TuneIn mobile app and website, asking TuneIn users to subscribe to Apple Music for the full experience. With more than 75 million monthly active users, TuneIn is the right place for Apple to try to attract people to subscribe to Apple Music.