Look for a printed code on a sticker inside the CD/DVD case or on the back of the manual.
Since version 1.15.1 is an older build, the best way to play it legally is through . product key for euro truck simulator 2 1.15.1
If you are running the older standalone 1.15.1 version without the Steam client, you must use the internal legacy activation process: Look for a printed code on a sticker