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();
 if( $args->theme_location == ‘nav-top’ )
        $items .= '<li>' . $searchform . '</li>';

    return $items;
}