How to activate one year free Apple TV 3.5 months after start using new iPhone 12 Pro Max

You cannot.

The window for activation is within 3 months of purchasing the eligible device.

If you failed to do that, you cannot redeem the offer any more.

Beyond that, Apple no longer offers the free year. Starting July 1st, the offer has been reduced to 3 months free.

You can try contacting Apple Support directly and pleading your case though, they may let you activate the offer. Click here: link-> https://offers.appletvapp.apple/tvp01213mf

and here: link-> Get Support

You can't. The offer expires if not started within 90 days.

(201917)