Product research is for when you already have a good understanding of your users, and you want to understand the problems they have using your actual product — how often they use it, where they get stuck, if there any obstacles or usability issues, or gaps in the product that you could potentially solve with new features.
tl;dr; Do not add discount code input to the checkout page. It will distract people from finishing the purchase and make them second-guess the decision. There are better ways to distribute promotions to new and existing customers, e.g. campaign URLs.