Search .onion version of DuckDuckGo by default in Tor browser.
DuckDuckGo has a .onion version, but it does not ship with Tor.
One of the more well known privacy tools is Tor, or The Onion Router. It anonymizes your traffic by bouncing it through many relays around the world. Certain websites allow secure access through Tor with a .onion version of their website where all traffic stays within Tor.
DuckDuckGo is a search engine that doesn't track you. They have a .onion version of their site, but it doesn't come bundled in the browser's choice of search engines.
I did the research to create and embed the XML so you can go to a single website in Tor browser and add it to your search bar.