This prequel DLC, whose events happen prior to "Stalingrad Campaign", completes the Eastern Front story as told by "Unity of Command". Take control of the Wehrmacht and its allies in Operation Barbarossa, from the dramatic early victories all the way to ignominious defeat at the gates of Moscow.
Open the Steam client, login and locate the Games tab on the top menu. Click this and then select “Activate a Product on Steam…”
Enter your unique code that's displayed on the 'Orders' page of CDKeys.com and follow the prompts to activate.
Your game is now viewable in the “Library” tab and is available to download/install when you’re ready. You can find additional support for the Steam client here: https://help.steampowered.com/en/wizard/HelpWithSteam