Adding the search function into the responsive main nav

I did this by adding this code in the functions.php of my child theme:

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;
}

You will need to modify your CSS some as well to prevent it from appearing on multiple lines:

form#searchform
{
	margin: 0.5em 0em;
	width: auto;
}

form#searchform .field
{
	width: auto;
}