I may be asking too much here, but is there any way to use an Air Canada promo code (like the ones they're giving away now) if I've reached the Air Canada site via Google Flights?

Obviously I want to plan out my itinerary on ITA and then google flights, so that I can get all the segments I want, but when Google flights then transfers me over to the AC site to actually buy the ticket, I'm past the screen where one normally types in the promo code.