Add external links in WP Admin Menus & Submenus

add_action('admin_menu', 'admin_menu_add_external_links_as_submenu');

function admin_menu_add_external_links_as_submenu() {
    global $submenu;

    $menu_slug = "externallink"; // used as "key" in menus
    $menu_pos = 1; // whatever position you want your menu to appear

    // create the top level menu
    add_menu_page( 'external_link', 'External Links', 'read', $menu_slug, '', 'dashicons-welcome-learn-more
', $menu_pos);

    // add the external links to the slug you used when adding the top level menu
    $submenu[$menu_slug][] = array('Example', 'manage_options', 'http://www.example.com/');
    $submenu[$menu_slug][] = array('Google', 'manage_options', 'https://www.google.com/');
}