Admin Panel – Custom Menu Sub-Item LINK

Insert the URL of the page as the $menu_slug argument. Also note that user levels are deprecated, you should pass a capability instead.

function add_custom_link() {
    add_submenu_page(
        'edit.php?post_type=cpt_custom',
        '',
        'Pending Posts',
        'edit_posts',
        'edit.php?post_type=cpt_custom&post_status=pending',
        ''
    );
}
add_action('admin_menu', 'add_custom_link');