I did it through the website and not the app, and it did not show up on the receipt.

I contacted GrubHub support and got:

TP - Ilse DT.: I understand how frustrating that can be, I apologize for the experience. It seems that the code was not applied, unfortunately, we are not able to apply it manually to the order but you can place a new one even using another restaurant if you wish.


So basically I can spend more money with them an try again. This whole promo is a sham and Square Enix should be ashamed of themselves.