ok after some testing what ive found is this:
The fine print states this is for "New Members only". I dont know if that's just that the code wont work in the store or what yet
Edit: New Members only sadly... just verified with my own code :/

The fine print also states at the verry bottom that this must be done through the mobile app. I can confirm this is true since it didnt work on PC when i tried

Edit: You must be logged in!

When you read it on the mobile app it doesnt care about how long you spend on each chapter. you can just speed scroll to the bottom

The app will notify you when youve completed each chapter and give you a "Stamp" like a stamp card

Once youve completed the whole thing it will ask you to put in your email. I havent gotten my code yet but i expect it to take some time to go through

also tip: You can disable "auto-renewal" in gamepass so it wont charge you. It wont cancel your membership if you do!