Note: After saving, you have to bypass your browser's cache to see the changes. Google Chrome, Firefox, Microsoft Edge and Safari: Hold down the ⇧ Shift key and click the Reload toolbar button. For details and instructions about other browsers, see Wikipedia:Bypass your cache.
// First wait for mediawiki.util to load, and the page to be ready.
$.when(mw.loader.using('mediawiki.util'), $.ready).then(function () {
	const title = mw.config.get('wgPageName');
	
	// Add link to toolbar.
	mw.util.addPortletLink(
		'p-tb', /* portletId */
		'https://en.wikipedia.org/wiki/Special:PrefixIndex/' + title, /* href */
		'Pages with prefix', /* text */
		't-prefixindex', /* id */
		'A list of all wiki pages with this prefix [⌃⌥i]', /* tooltip */
		'i' /* accesskey */
		/* '#t-recentchangeslinked' nextnode */
	);
});