How do you use promo codes to attribute visits?

The code works if it is short, it is on the ad and the floor asks for it when they take payment, with an end date. A standing discount that eats the margin and that the customer mangles does not work. Pick a word that is easy to say and the gap you want to fill, not a discount for the Friday that is already full. Tell whoever takes payment. Nobody on the floor knows the code and the customer gets angry at the bill. The ad attributes zero and the visit costs you. The code on the ad, the note at the till and the week's count of how many people said it. If nobody says it, it comes off; you do not insist with another percentage.