First, locate and click on the game: "American Truck Simulator" in your library




After that, press the "Play" button




A popup will be displayed, select "Play American Truck Simulator" or another desired option, and then click on the "Play" Button.



Note: You can mark the checkbox "Always use this option" to skip the popup in the next time you launch the game. Those same options are located in the properties window of the game.


Congratulations, You've opened the game.