Enable this extension by adding this to LocalSettings.php:

    wfLoadExtension('FlarumAuth');
    $wgGroupPermissions['*']['createaccount'] = false;
    $wgGroupPermissions['sysop']['createaccount'] = false;
    $wgGroupPermissions['*']['autocreateaccount'] = true;

Define the Flarum URL:

    $wgFlarumUrl = 'https://flarum.example.com';