To get an extended free trial period for Apple TV+, you have to
- not claimed this offer before,
- purchase your new device from Apple or an Apple Authorized reseller,
- claim the offer on your new eligible device, not on a different device,
- redeem the offer within 3 months of activating your new eligible device.
- For devices activated (first use) after July 1st, the offer period is shorter.
- The offer is for the first user (by Apple ID) of the device.
If you have been subscribed (payed) to Apple TV+ before, then the automated redeem method may not work for you. Then contact the Apple Support staff to see about your options.
Read here how to redeem: Apple TV+ offer - Apple
Read here about the conditions: Apple TV+ offer - Terms and conditions - Apple
Read here if you have trouble redeeming: If you can't redeem your promotional trial of Apple TV+ - Apple Support