I tried adding a link to MediaWiki:FooterLinks as suggested by this doc but it doesn't seem to have worked

It adds the text, but it doesn't get parsed into HTML

		$this->getSkin()->addToSidebarPlain(
			$groups,
			$this->getSkin()->msg( 'FooterLinks' )->inContentLanguage()->plain()
		);

But with the addToSidebarPlain call, it means the syntax is... odd like the usual MW sidebar syntax :)

See https://www.mediawiki.org/wiki/Manual:Interface/Sidebar#Customize_the_sidebar

So for example, https://api.wikimedia.beta.wmflabs.org/w/index.php?title=MediaWiki:FooterLinks&oldid=61 works

** foundation:|Test using the foundation IW prefix as per https://api.wikimedia.beta.wmflabs.org/wiki/Special:Interwiki