The offer from Apple is for a 3 months free trial to Apple TV+ for users that recently purchased a new iPhone, iPad, iPod Touch, Apple TV box, or Mac, and haven’t enjoyed an extended free trial for this service before. Redeem the offer within the first 90 days. Redeem this offer on your new eligible Apple device, not on a different device. Redeem the offer in the TV app, find the “Enjoy 3 Months Free” button there. The offer is for the first user of the new device, which may not always match the purchaser (e.g. gifts).
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
For Macs, users have to additionally sign in to the TV app (menu Account), for the redeem option to appear.