It is something you negotiate before everything starts. It cannot be done afterwards. It is better if you have a track record of adding value and a company that has the right spirit of risk reward.
Everything we design is intended to boost sales, nothing unique about that. It is about who assumes the risk. If they are paying all the bills and paying what you asked for, as well the full business costs, then there is no risk on your part. You get to enjoy seeing the success of your design succeed, hopefully. If you structure your next deal so that you can share risks, it is easier to ask for a reward.