How do I get my 1year free Apple TV subscription with purchase of iMac

Did you buy the iMac within last 3 months?

If yes, read here => https://offers.appletvapp.apple

Note: Only valid if you haven't already subscribed.

The TV app on Mac has a separate sign in (menu Account). You probably need to sign in before the “Enjoy 1 Year Free” offer will show in the app.