If you have had access to an Apple TV+ extended free trial before, then you may not be eligible for a new offer.
Else for Macs, see the last line below.
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
↳ Step 5 says to contact Apple if the prior steps didn’t bring a solution. They can assist.
Note that for Macs, users have to additionally sign in to the TV app (menu Account), for the redeem option to appear.