WordPress custom plugin developement

Use add_submenu_page and set $parent_slug to null:

add_action('admin_menu', 'wpdocs_register_my_custom_submenu_page');

function wpdocs_register_my_custom_submenu_page() {
    add_submenu_page( 
        null,
        'My Custom Submenu Page',
        'My Custom Submenu Page',
        'manage_options',
        'my-custom-submenu-page',
        'my_custom_submenu_page_callback',
    );
}