I might be misunderstanding, so please correct me if I’m wrong.
The is_external setting has to be ticked if the search method used is not the forum search, like Bing or Google. This is so that the component will redirect the user accordingly.
You can change the search_url setting to bing.com/search?q=%{selected_text} to search via Bing. As long as the search engine has a way to interpolate the search query, you can type it in.