/**
 *  Menu items - Add "Custom sub-menu" in menu item render output
 *  if menu item has class "menu-item-target"
 */
add_filter( 'walker_nav_menu_start_el', 'wpdocs_menu_item_custom_output', 10, 4 );
function wpdocs_menu_item_custom_output( $item_output, $item, $depth, $args ) {

    $menu_item_classes = $item->classes;
    if ( empty( $menu_item_classes ) || !in_array( 'menu-item-target', $menu_item_classes ) ) {
        return $item_output;
    }

    ob_start(); 
    ?>
    <ul class="custom-sub-menu">
        <li class="custom-menu-item">
            <a href="#custom-url">
                <?php _e( 'My text', 'wpdocs' ); ?>
            </a>
        </li>
    </ul>
    <?php
    $custom_sub_menu_html = ob_get_clean();

    // Append after <a> element of the menu item targeted
    $item_output .= $custom_sub_menu_html;

    return $item_output;
}