Sorry, but how do you actually turn this on?
I installed Tampermonkey and copied the script in and the dashboard says it's enabled, but Tampermonkey says no scripts are running and a search on DuckDuckGo just searches on DuckDuckGo.

Only the pre-defined search engines that come with Opera can be set as the default one.

@silas229 Except, maybe, for Edge/IE and Chrome, almost all browsers get their revenues from searches and from pre-added bookmarks/speed dials.