You can purchase a direct license, which provides a product key that can be used to activate the game.
A product key is a unique 25-character code used to verify your ownership of the game. You can obtain one through several official channels: Product Key For Euro Truck Simulator 2 Version 1.35.1.31
In the past, before the Steam version became standard, ETS2 was sold as a physical copy. These came with a product key, often in the format XXXXX-XXXXX-XXXXX-XXXXX-XXXXX (five groups of five characters). You can purchase a direct license, which provides