For anyone reading this thinking "what a killer feature I need to switch to DDG now". All browsers do this natively now. In Chromium engines it's found under "Manage Search Engine". In Firefox it's under "One Click Search Engine".
For example, just used HN's search bar, and there it showed up in the options ready to customise keywords to my liking (if I wanted to change it).
Before anybody claims it's an additional step, it's not. DDG requires you to know what the keyword is before you use it, which is the same as having to use the search on the site before. So I guess I agree it's a killer feature if you exclusively use Firefox for the time being.
For example if I want to want search wikipedia, I type "en.w" and hit tab.