Applying a code to a past appointment

I am working with a client who has their customers buy packages at times for a set number of sessions. They get a code to enter and are supposed to use it as they book all sessions in the package. They forget sometimes and will book without the code. How can I go in after the fact and apply the code? Acuity tells me it not valid when I try to edit an appointment that has occurred already. 

Bummer that no one answered this.  😞