The product key is your proof of ownership for the base game. You do not need a new or different key. You just need to locate the one you already have.
Creating a account without a manual product key is standard for players who own Euro Truck Simulator 2 (ETS2) or American Truck Simulator (ATS) via Steam . Instead of a key, you use Steam OAuth to verify ownership . The Steam Connection Method
Creating a World of Trucks account does for the vast majority of players. The key‑based registration method is a legacy option intended for physical boxed copies, while the modern, convenient, and recommended approach uses your existing Steam account as proof of game ownership.