The format of your key depends on the version you purchased:

: If no internet is available, the game generates an activation.scb file in your Documents/Euro Truck Simulator 2 folder. You can upload this file to the Official Offline Activation Page using another device to receive an activation code. Critical Security Warning Support - Euro Truck Simulator 2

It sounds like you’re looking for a specifically for Euro Truck Simulator 2 version 1.4048 — possibly from a listing labeled “exclusive.”

You can download a free trial version on Steam to see if your PC handles the latest updates before buying.