The Epiphany web browser does not have a handy search bar like Firefox. But its powerful smart bookmarks system lets you create as many as you like for any search engine you choose.
Smart bookmarks are bookmarks which can take arguments. To create one for use as a search box, start by doing a search in your preferred search engine. Then select Bookmarks->Add Bookmark to create a new bookmark of the results page and edit the address to replace the search terms with %s.