In Firefox, there is a way that doesn't require a flag: create a bookmark with the url, and set its "keyword" field to the trigger word you want for the engine.
That works in chrome/edge too. (Even better, Edge matches urls with %s very good, and the URL bar offers past searches as suggestions better than Firefox does.)
They removed most bookmarks and search engine settings, but bookmarks with keywords work fine at the moment, for both mayor desktop browsers.
Also you can store them in a file(!) and import them for a nice cross browser experience, keeping track of them, and not losing them.
