There is such a search box. On desktop, click on "Add" in the sidebar. On mobile, you'll have to open the hamburger menu. There's also URL input for articles/content types I haven't added search for.
Thanks for the reply - that's great. I'm on mobile and looked for 'search', but now I see how to do it.
I thought about calling it "search" but went for "add" since that is technically more accurate with the URL inputs--and it's already a fairly common occurrence for people to enter search terms in those instead of URLs. Though your comment makes me thing that maybe I should just call it "search"...
Another option could be to add a search box to the landing page which when submitted takes you to the 'add' page
I'd thought about putting the search bar on the main page but wasn't sure how to make the UX good. It never occurred to me to just switch the page. I'll probably do that, thanks.