We're looking for this funcitonality as well. Browsing the API docs it doesn't look like it's something we can hook into. I'm hopeful someone can chime in with how I'm wrong haha. I was looking at the "Create an order" section on the API documentation and saw that there's a "discountLines" entry if you make an order through the API. Do you know if that has to be a preexisting discount code to be used for that? Because if not, I wonder if that would be a way to work around the limitation.