Add Search as a list item in the Primary Nav

add_filter( 'wp_nav_menu_items', 'custom_menu_extras', 10, 2 );
/**
 * Filter menu items to append Search form to Prmary Nav.
 *
 * @param string $menu HTML string of list items.
 * @param object $args Menu arguments.
 *
 * @return string modified menu.
 */
function custom_menu_extras( $menu, $args ) {
    if ( 'primary' !== $args->theme_location ) {
        return $menu;
    }

    $extras = '<li class="menu-item search">' . get_search_form( false ) . '</li>';

    return $menu . $extras;
}