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.
Use your key on Epic Games launcher to download your game
Rebel Galaxy
To activate your game with Epic Games, log in to your account on their website, click on "Redeem Code" at the top right of the page, and then enter the game's CD key or code in the field provided. Click "Redeem" to verify the code and add the game to your Epic Games game library. You should now be able to download and play the game from your library.
Use your code on the Xbox Live network to activate your games or increase your Xbox Live account's credit. To access the Xbox Live network, you need the Xbox One, a broadband connexion, a Xbox Live account, and a storage device according to your console: Hard disk drive, memory card, or USB drive.
Xbox One UNITED STATES
Localization restrictions applied. This product can be activated only in the United States of America.