If this doesn't help, open a AMD Rewards Support ticket from here: Support - AMD Rewards and they should be able to give you instructions on how to get the XboxX Game Pass.
From XboxX Game site: Xbox Game Pass for PC (Beta) | Xbox
At a Reddit Thread asking the same question, One User said to refer to the Retailer and the other said XboxX coupon code was written on his receipt.
But it probably will be best just to open a AMD Reward Support ticket to find out if the above doesn't work.
