How can I set up my one year free Apple TV?

If necessary, you are entitled to 90 days of free telephone support from Apple. Try contacting them.

Apple Support Contact.    

Work through the screens to get an email or chat option.

Apple Support by e-mail or chat.    

See the four-step guide at Apple TV+ - One Year Offer - Apple

Use the notification you received, and See Set up Apple TV https://support.apple.com/guide/tv/set-up-apple-tv-atvb73e46488/tvos