Add custom li or menu to wordpress navigation

/**
 * Hardcodes shop item in navigation
 * @param string $items HTML with navigation items
 * @param object $args navigation menu arguments
 * @return string all navigation items HTML
 */
function new_nav_menu_items($items, $args) {
    if($args->theme_location == 'main-menu'){
       $shop_item = '<li class="spec"><a href="#" title="title">title</a></li>';
       $items = $items . $shop_item;
    }

    return $items;
}
add_filter('wp_nav_menu_items', 'new_nav_menu_items', 10, 2);