Mafia III: Definitive Edition includes the main game, all Story DLC (Faster, Baby!, Stones Unturned, Sign of the Times) and Bonus Packs (Family Kick-Back Pack and Judge, Jury & Executioner Weapons Pack) bundled in one place for the first time.
Go to https://www.xbox.com
Login to your account
Hover over your account link (the top right of the screen) a menu will appear, click on the "Redeem Code" button.
On the next page, click "Redeem"
Enter your code
Click "Confirm" again to assign the key to your account.