Add Search Form To A WordPress Menu
<?php
//Add Search Form To A WordPress Menu
add_filter('wp_nav_menu_items', 'add_search_form', 10, 2);
function add_search_form($items, $args) {
if( $args->theme_location == 'CHANGE-THIS-TO-YOUR-MENU-NAME' )
$items .= '<li class="search">
<form role="search" method="get" id="searchform" action="'.home_url( '/' ).'">
<input type="text" value="search" name="s" id="s" />
<input type="submit" id="searchsubmit" value="'. __('Search') .'" />
</form>
</li>';
return $items;
}