IDK if it's going to be different for xbox but on PC :
Prime Gaming Scroll through the games and then "claim" the ones you want and/or the cosmetics and bonus packs. You usually have to go through the step of linking your accounts but it's pretty easy/self-explanatory. Also, Amazon is offering games via Luna for prime members and they're doing a lot of mobile and play on 'any platform' games as well.
I check in from time to time because they usually have a game or two that's worth claiming. Like I went last night because I figured they'd have a cosmetic pack for AC Ragnarok (they did

).
ETA: I think it's the same website for all platforms.