What device purchase could give you this offer? The offer for Apple TV+ from Apple is for buying a new iPhone, iPad, iPod touch, Apple TV box, or Mac, from Apple or from an Apple Authorized Reseller.
The offer is for the first user of that eligible device.
The offer should be redeemed within the first 90 days.
The offer should be redeemed on that new eligible Apple device, even if you received that message on a different device.
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 promotion for an Apple subscription - Apple Support
Note that for Macs, users have to additionally sign in to the TV app (menu Account), for the redeem option to appear.