You need to get an Apple I.D. setup with Apple first. Go to "https://support.apple.com/apple-id".At the lower right of your screen select "Create your Apple ID". Follow the screen prompts create your ID and enter the info as noted'. At some point you will need to provide credit card info. But don't worry you can cancel at any time. If your concerned just cancel AppleTV+ a week or two ahead of trial expiration date. Note you will need to setup Itunes as well.
Lastly, If you buy some recent UHD & Blu-ray titles you will find that some of the digital redemption codes are for Itunes only.
A deadline date is unclear, but I would expect it to end by 12/31/2021.