If you have a DVD-version of the game, then do the following:
1. Register or login (if you had already registered) at https://store.gaijin.net/
2. Activate the game here.
3. Download and log in to the Yuplay-client and launch the installed game.
If you have a digital version of the game purchased at https://store.gaijin.net/ , then you do not need any activation code. You need to log in to the Yuplay-client and run the game launcher.