If you do not have a Steam account, you will need to download the client from the official website, install it and then create your account. Once you are logged into your account, click on "Add game" in the bottom left corner and then on "Activate product on Steam". After a few clicks on "Next", you can finally enter your CD key, the game will automatically be added to your library and the download will begin.
If you have a digital Xbox Play Anywhere title, you can play on both Windows 10 PC and Xbox One.
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Play Anywhere games can be played on your PC and your Xbox. You will need to have installed the Windows 10 Anniversary Edition update on your PC, as well as the latest update on your Xbox One console. You only have to log into your Xbox Live/Microsoft account and your Xbox Play Anywhere games will be available to download. On your Xbox One, games will show as "Ready to Install" under "My Games and Apps". On your Windows 10 PC, games will appear in "My Library."
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CD key to use on the GOG GALAXY client to download your game.
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To activate a game you download on GOG Galaxy, open the application on your computer, log in to your account, click on the menu icon, and then select "Game Codes". Enter the game code in the appropriate field and click "Continue". Once the code is verified, the game will automatically be added to your GOG Galaxy library where you can download and play it.
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