The key you have received needs to be activated on Steam. This can be done in a few simple steps:
Create a Steam account
Steam is the world's largest digital distribution platform for games and software.
If you haven't done it already, you will need to create a new Steam account.
It's free and doesn't require any credit card details.
Go to the Steam website and click on the 'Install Steam' button.
Install and launch the Steam client and create a new Steam account.
Activate the game on Steam
1. Sign in with your Steam account in the Steam client.
2. Click on '+ADD A GAME' in the bottom left corner of the Steam window.
3. A small menu will appear. Click on 'Activate a Product on Steam'.
4. Follow the activation instructions in the Product Activation window and enter the code on the screen shown below.
Steam will ask you if you want to install the game after a successful activation (doesn't apply to downloadable content).
Play the game
1. Go to your Steam library (located in the top menu of the Steam client).
2. Choose the game you have activated (American Truck Simulator or Euro Truck Simulator 2) in the games list and click on Play.