add_filter('wp_nav_menu_items','add_search_box', 10, 3);
function add_search_box($items, $args) {
 ob_start();
 get_search_form();
 $searchform = ob_get_contents();
 ob_end_clean();
  $items .= '<li class="search_menu"><span class="fa fa-search"></span><ul class="sub-menu"><li class="search_item"></li></ul></li>';
 return $items;
}