Pages such as https://meta.wikimedia.org/wiki/Timeless/Newsletter have a "site" text input. The autocomplete behavior for this text input is kind of wonky, though. It appears to only be doing prefix matching, so typing "wikipedia" doesn't work. If you enter "en", it's a bit better.
Similarly, you can't enter "mediawiki.org" and have it work. Instead, you try to submit and it goes "nope!" because it has to be "www.mediawiki.org" apparently. This may be a distinct issue. Feel free to split this issue if you think that's appropriate.