FIRST STEP:
Before to launch the game, you need to know your code and how? Easy, see my image example. :-)

1. Go to on your Steam library
2. Click on DOOM 3 with right mouse
3. Manage (gestisci)
4. Product codes (codici prodotto)

SECOND STEP:
After seeing your code, you can open the game.
Once you open the game, that window will pop up and tell you to enter the code. Remember that the above is practically already written, both the first on the left and last two letters on the right. You will only have to write below, but in small and not great letters. Take a good look at the passages of these images.
1.
2.
3.

DO NOT PASS YOUR CODE TO SOMEONE, IS UNIQUE AND ONLY FOR YOU!
THANK YOU!