The offer is available to new subscribers that recently bought an iPad (or iPhone, Mac, Apple TV box) from Apple or from an Apple Authorized Reseller. The offer should be redeemed in the first 90 days, after first activating the new eligible device. No link is needed. If your account and device qualify, then an “Enjoy 3 Months Free” button should appear in the TV app on that new iPad.
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.