Cacher is the code snippet organizer for pro developers

add_filter('wp_nav_menu_items', 'add_search_form', 10, 2);
function add_search_form($items, $args) {
if( $args->theme_location == '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="'. esc_attr__('Search') .'" /></form></li>';
        return $items;
}