This is hax :)

1. Debug and find __MY_VARIABLE__SLUG__
   1. die(var_dump($submenu));
2. Search for right slug name

function add_custom_link_into_submenu() {
		global $submenu;
		$custom_url= 'https://google.com';
		$submenu["__MY_VARIABLE__SLUG__"][] = array( Click my link', 'manage_options', $custom_url );
}

 and register it inside init()

add_action('admin_menu', array($this,'add_custom_link_into_submenu'));