While coupons, promo codes, etc. cannot be automatically applied during online registration, you can utilize a few Jackrabbit features to get the job done!
From the Tools menu > Edit Settings > User-defined Fields, label a Family User-Defined field as "Enter Coupon#". This will provide a text box on the Registration Form where the customer can enter the information.
Use Search from the Families menu to report on Family User-defined Fields by changing the Display Setting Show Family User-defined fields? to Yes.
Using the Pay links in the Search Families Report results, you will then adjust the posted fees by entering credits on the families' accounts using the instructions in the the Help section Post a Non-Monetary Credit to an Account (Family).