Just bought a new MacMini. How do I claim my 3 months of free Apple TV ?

See the following from https://offers.appletvapp.apple/tvp01213mf

Redeem 3 months free of Apple TV+ now.

You can start your 3 months free subscription to Apple TV+ after setting up your new Apple product. Here's how to redeem your offer:

1 - Turn on your new Apple device and sign in with your Apple ID.

2 - Open the Apple TV app. Make sure your device is running the latest version of iOS, iPadOS, tvOS, or macOS.

3 -The offer should be presented immediately after launching the app. If not, go to the Apple TV+ tab where you'll see the offer.

4 - Tap Enjoy 3 Months Free.

Check Eligibility

There is no code needed. Your free trial subscription is linked to your Apple ID -- the ID on your new device.

For Macs, remember to first sign in to the TV app (menu Account). Then the redeem button “Enjoy 3 Months Free” should load for the eligible account.