From the How to Redeem section:
- Make sure your iPhone or iPad is running iOS 15 or later.
- Pair your eligible audio device to your iPhone or iPad.
- Open the Apple Music app on your iPhone or iPad and sign in with your Apple ID. If the offer doesn’t appear immediately after launching the app, go to the Listen Now tab where it will appear.
- Tap Get 6 months free.
Note that iOS 15 is required.