How to redeem Apple tv+ 1 year subscription when you purchase a new Mac?
Here's an answer from Apple's official promotional document.


  Redeem your 1 year free in the Apple TV app starting November 1, 2019. Make
  sure your eligible device is running the latest iOS, iPadOS, tvOS, or macOS.
  
  
    Offer must be claimed in the Apple TV app within 3 months after first setting
    up your new device. To see the offer appear, you will need to sign in with your
    Apple ID on your new device. If you’ve purchased your new device before the
    launch of Apple TV+ on November 1, you will have 3 months starting November
    1 to redeem