Another approach for Firefox (not sure about other browsers): Right click on the search box on a website and 'Add a keyword for this search', save it then edit

Another approach for Firefox (not sure about other browsers): Right click on the search box on a website and 'Add a keyword for this search', save it then edit the bookmark to end up with something like https://duckduckgo.com/?t=h_&q=-site:example.com https://duckduckgo.com/?t=h_&q=-site:example.com %s