@estanis you cannot set it to be the default, though. Ie. it won't be used when you type in "whatever" on the URL bar, unless you prefix it with the shortcut set there (like "b" for "Bing" or "z" for "Amazon").
See the linked forum post for my details.
And as Opera isn't customizable enough (in that area), I basically dumped it and switched to Vivaldi. Opera is (for default search) in the same boat as Safari… 😕