The offer has the condition that it has to be redeemed within the first three months.
The offer has to be redeemed on your new eligible Apple device in the TV app. Once you have the subscription, the service will be available on any of your devices (when signed in with that same Apple ID account).
For Macs, you should additionally sign in to the TV app (menu Account), then the redeem button will load (for eligible accounts only).
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