To get a product key for Euro Truck Simulator 2 (ETS2) version 1.15.1, you generally need a valid license from a previous purchase or a new official copy. Note that version 1.15.1 is an older "legacy" update from late 2014. store.steampowered.com How to Find Your Product Key
The 1.15.1 update introduced several new features and fixes to the game: ETS2 1.15.1 | New patch (Euro Truck Simulator 2) YouTube• Dec 6, 2014 product key for euro truck simulator 2 1.15.1