functions.php

add_filter( 'wp_nav_menu_items', 'your_custom_menu_item', 10, 2 );
function your_custom_menu_item ( $items, $args ) {
    if ($args->theme_location == 'header') {
        $items .= '<li class="menu-item"><a id="openOptionsSidebar" class="shifter-handle">More</a></li>';
    }
    return $items;
}