Mage2\Order\Module::registerAdminMenu PHP Method

    public function registerAdminMenu()
    {
        $adminMenu = ['sale' => ['label' => 'Sales', 'route' => '#']];
        AdminMenu::registerMenu('mage2-order', $adminMenu);
        $adminMenu = ['sale' => ['submenu' => ['order' => ['label' => 'Orders', 'route' => 'admin.order.index']]]];
        AdminMenu::registerMenu('mage2-order', $adminMenu);
        $orderStatusMenu = ['sale' => ['submenu' => ['order-status' => ['label' => 'Order Status', 'route' => 'admin.order-status.index']]]];
        AdminMenu::registerMenu('mage2-order', $orderStatusMenu);
    }