If you're looking for a way to get FIFA 21 for free on Xbox, there are a few options available. One option is to sign up for an EA Access subscription. With EA Access, you'll be able to play FIFA 21 for 10 hours before launch. If you decide to purchase the game after your 10 hours are up, your progress will carry over.
Another option is to wait until FIFA 21 is released on Xbox Game Pass. Xbox Game Pass is a subscription service that gives you access to a library of games. FIFA 21 will be added to the Xbox Game Pass library on launch day, so you'll be able to play it for as long as you're a subscriber.
Finally, you could try to find a physical copy of FIFA 21 on release day. If you're lucky, you may be able to find a store that's giving away copies of the game for free. However, this is unlikely and you'll probably have better luck finding a cheap deal on a used copy.