Even if the search form uses POST, the server does read the parameter if it's specified in the querystring. That is, https://duckduckgo.com/lite/?q=asdf https:/

Even if the search form uses POST, the server does read the parameter if it's specified in the querystring. That is, https://duckduckgo.com/lite/?q=asdf https://duckduckgo.com/lite/?q=asdf works, so adding it as a search engine with URL .../?q=%s should work as well.