http://www.asus.com/event/2016/rog/doom/index.aspx
under the FAQ tab
"Where can I get the ASUS Code?
You will be given an ASUS code when you buy eligible products from participating stores."
so apparently your seller was suppose to give you the code but your not out of luck, the following is in the FAQ info on how to get a code.
" If you can’t find or didn’t receive your ASUS code, please contact your local ASUS representative and provide the following information:
1. Email subject title: DOOM
2. Name of the store where you made your purchase
3. Name of product purchased
4. Attach a scanned copy of your receipt"
the email address you will need to send the info to will depend on where you live and they are listed down the page under redeem your game key, is a link that says "contact ASUS local team" find the country where you live and send them an email with the info above. its in there hands.
good luck (also my browser didn't like that web site I had to use internet explorer to view it apparently ASUS went cheap in the web compatibility department.)