@switchingsocial As others have suggested, instead of using the built-in search box's iframe, you could copy its design (or create one of your own) and then use JS to adapt the submitted form's URL params to your needs.

@josemanuel

I'm not a developer, that's why I'm asking for help :)

I can just about add a simple HTML form, but I'm not sure how to have both the user's search terms and the "drm-free" text combined into one set of results.