Buy Age of Empires III: Definitive Edition - Knights of the Mediterranean

For Steam CD Keys, you must download (HERE), install the Steam software and create a new account. If you already have a Steam account, once you log in, click "Add a game" (located in the bottom left corner), and then "Activate a Product on Steam". After a few more clicks, you will be prompted to type in your game code. Thus, the game will be activated and automatically added to your account library.

You need a Microsoft account to download your game. Create one or log into yours if you already have one. Follow the steps on the Microsoft app to buy the game and it will be automatically added to your account. It will appear in "My Library" and you can access it anytime.