Customize search engine in Thunderbird 45

I want to add DuckDuckGo as a search engine in Thunderbird 45. How can I do that? I looked at https://support.mozilla.org/en-US/kb/open-search but it seems to be outdated. I also looked at https://duck.co/help/desktop/thunderbird which seems to be based on the Mozilla instructions from https://support.mozilla.org/en-US/kb/add-search-engine-open-search . I tried the steps on Thunderbird 45 for Windows but they didn't work: DuckDuckGo wasn't listed as a search engine in the preferences where you can choose the search engine. To make sure that I followed the steps correctly, I tried them on Thunderbird 38.6.0 from Ubuntu 16.04 where they worked flawlessly.