add_options_page( $page_title, $menu_title, $capability, $menu_slug, $function);
add_options_page( 'Login', 'Login Page', 'manage_options', 'loop.php?url=".esc_url($login_url)."');
You’re trying to feed a URL to an option that takes only a slug.
$menu_slug
(string) (required) The slug name to refer to this menu by (should be unique for this menu).
Default: None
from http://codex.wordpress.org/Function_Reference/add_options_page
You can easily add a link to the admin bar at the top of the screen:
add_action( 'admin_bar_menu', 'toolbar_link_to_mypage', 999 );
function toolbar_link_to_mypage( $wp_admin_bar ) {
$args = array(
'id' => 'my_page',
'title' => 'My Page',
'href' => 'http://mysite.com/my-page/',
'meta' => array( 'class' => 'my-toolbar-page' )
);
$wp_admin_bar->add_node( $args );
}
( https://wordpress.org/support/topic/adding-a-admin-menu-item-that-links-to-a-url )
I’ve been going through http://codex.wordpress.org/Administration_Menus and it appears that you might not be able to hardcode a link in the admin menu. Every function requires a slug
to an existing page.
Could you create an options page and have the link there instead?