Call of Duty: Black Ops IV is a payed game, which you need to purchase as a disc (physically) for your Xbox One/One S/Series X or digitally on the Microsoft Store, for any Xbox One/Series X|S console.
From the App/website, players must locate the section "CALL OF DUTY," where they'll be able to choose from five different COD-themed menus. Once players purchase the menu via the App/website, they will be prompted to provide an e-mail address, where they'll receive the code to unlock the Burger Town skin and 2x EXP.