Hey hey! There should be a link in that email you got. All you have to do is follow that link to redeem your copy of Beat Saber to your account! (There isn't a code, just the link.) Then you should be able to download it onto your headset and get in the game!
If you have any trouble redeeming it, you'll want to reach out to the Support Team for help: https://store.facebook.com/help/support/quest/