I cant see this of much use to most users, whereas a text field option in the build text field would be far more useful/beneficial, so it could be used as for example entering a discount code e.g. FRIDAY20, then a calculation can be formulated that when that specific term is entered a calculation of the discount is applied/displayed etc.
You see, this feature is meant for the cases when you need to show the text in the results instead of the number (for example, calculate the T-Shire size based on the entered parameters (height, waist, etc).
Our users were asking for this feature and we’ve released it based on the comments from this thread.
However, I completely get your idea and agree that a field to apply a discount coupon would be just great. If your request gets more votes, we’ll try to think it over in the future updates