Want to automatically add a search box to your nav? Here’s how:

 
add_filter('wp_nav_menu_items','add_search_box', 10, 2); 
function add_search_box($items, $args) { 
ob_start(); 
get_search_form(); 
$searchform = ob_get_contents(); 
ob_end_clean(); 
$items .= '<li>' . $searchform . '</li>'; 
 
return $items; 
}