I assume this is the offer you are referring to for returning customer who have not used the service in the last 12 months:
This offer is for new and qualified returning subscribers only. If you are not currently a subscriber to Apple Music, and you have not previously subscribed to Apple Music or Apple One within the last 12 months or had access to Apple Music through a Family Plan within the last 12 months, you are eligible for this offer.
https://offers.applemusic.apple/en-ph/six-month-music-offer
You would then see the offer in the Home tab of the Apple Music app if not presented on the screen, where you redeem by tapping the "Accept Now" button.