From the FAQ:

I already have Apple Music. Can I redeem this offer?

This offer is valid only for new and qualified returning subscribers to Apple Music.

If eligible, here’s how to redeem your offer:

1.Find the Apple Music app from your PS5 console’s search bar, or find it under “All apps” in Media home.

2.Download and open the Apple Music app and follow the on-screen instructions.

3.Sign in with your Apple ID or create an Apple ID if you don’t already have one.

4.Enjoy up to 6 free months of Apple Music.