I bought a new iMac how do I Install Free subscription For a year?

Go to > https://www.apple.com/apple-tv-plus/ and click on "Check eligibility"

Also please note, the subscription is for Apple TV + and NOT all the content in the Apple TV app. Also, if you are already a subscriber you will not get an additional year. Finally, you have to redeem the offer within 90 days of your original purchase of the qualifying product. If you meet the qualifications, enjoy, Apple TV+ has some really good content and the library is growing all the time.