Promo code MC50 offer

During the online purchase process of my new Xfinity internet service there was a pop up screen offer for a $50 card if I purchased that day. I did. But there was no place to enter the code. I was told over the phone that they would make a note concerning this on my account. I also spoke again today with Xfinity about it but nobody seems to have heard of MC50 although many users in this forum have had the same problem as I’m having. 

please help

thanks