The UI for this feature barely even exists in Firefox -- you have to make a bookmark, and then edit it to replace the query with %s and add a keyword. There is also the half baked "one-click search engines" thing, but the button to "find more" just redirects to the extension store.

The Chrome UI is somewhat better and it also automatically creates these when you use a search somewhere, with a keyword equivalent to the site's domain.

I don't know why Mozilla doesn't just copy this old Opera UI: https://i.postimg.cc/1XCg1HG8/opera-search-edit.png