I think I found the info. This is the link to the pdf. If read through and click/scroll to the Crossfire section, it details how it is done.
http://sites.amd.com/us/Documents/Never_Settle_Reloaded_Terms_Conditions.pdf
To obtain copies of the Free Games, Participant must purchase two eligible AMD products (see
"Eligible AMD Products" below) in a single transaction from a participating retailer and obtain from the
participating retailer an AMD Unique ID. The AMD Unique ID is submitted on the
http://www.amd.com/neversettlepromo site to obtain the Steam® Key Code for Sleeping Dogs, Hitman
Absolution, Tomb Raider and BioShock Infinite, an EA Origin® Key Code for Crysis 3 and a Uplay Key Code for Far
Cry 3 (collectively, “Key Codes”). The Key Codes are redeemed at www.steampowered.com/getsteam,
www.store.origin.com, and http://uplay.ubi.com/.
Promoter:
To Receive AMD Unique ID: Participant should refer to the participating retailer for instructions regarding
obtaining AMD Unique ID before purchase. As examples only, a participating retailer may require you to request
AMD Unique ID from the cashier upon purchasing the Eligible AMD Products; a voucher with the AMD Unique ID
may be inserted into the shipping box; AMD Unique ID may be emailed to you after the participating retailer
confirms your online purchase; or a participating retailer may require you to fill out an online request form after
your purchase demonstrating that you purchased the Eligible AMD Products from that participating retailer during
the Promotion Period. The AMD Unique ID may be given to you on a physical coupon voucher or it may be
provided to you electronically.