How do I add an "At Sign Search" to Firefox 68?

Firefox has a really neat "at-sign search" feature where, for some search engines, if you preface a search with @[engine_tag], the search will take place on that site instead of in the default search engine. It seems like the documentation on that feature is lacking on Mozilla's site - where in the Firefox preferences can these tags be set? It seems like the "keyword" portion of the search preferences sort of fills this role, but it doesn't have the autocomplete features of the at-sign search. Pic links below to show what I'm talking about:

List of all "at sign search engines":

Typing in an "at sign search engine" name:

Searching the engine:

Result of the search:

Solution 1:

The search engine list is available in the settings under the Search category. Below the list of available search engines is a link to find more which will take you to the search tools section of the add-on's website. Adding and removing search engines is documented on the support site under Add or remove a search engine in Firefox.

After adding a search engine to the list of available ones, you can add it to the @ list by creating a keyword for it that begins with @. This is also done in the preferences area search engine list by double-clicking the keyword column next to the search engine and entering the desired word. This is documented at Assign shortcuts to search engines

If you're looking to make your own search engine available for your site, that can be done via the OpenSearch description format.