protected $_coreRegistryl;
protected $_checkoutSession;
protected $_remote;
protected $orderFactory;
protected $_objectManager;

public function __construct(
    MagentoBackendBlockTemplateContext $context,
    MagentoFrameworkRegistry $registry,
    MagentoCheckoutModelSession $checkoutSession,
    MagentoFrameworkDataFormFactory $formFactory,
    MagentoSalesModelOrder $order,
    MagentoFrameworkObjectManagerInterface $om,
    MagentoSalesModelOrderFactory $orderFactory,
    MagentoFrameworkHTTPPhpEnvironmentRemoteAddress $remote
        ) {
    $this->_coreRegistry = $registry;
    $this->_checkoutSession = $checkoutSession;
    $this->_order = $order;
    $this->_objectManager = $om;
    $this->_remote = $remote;
    $this->orderFactory = $orderFactory;
    parent::__construct($context, $registry, $formFactory);
}

public function getOrder()
{
    $orderId = $this->_checkoutSession->getLastOrderId();
    return $this->orderFactory->create()->load($orderId);
}

protected function _prepareForm()
{
    $model = $this->getModel();
    $order = $this->getOrder();
    if($order->getId() && $this->_checkoutSession->getData('session_check') == '1') {
        /** @var MagentoFrameworkDataForm $form */
        $form = $this->_formFactory->create( .....

class Success extends MagentoCheckoutControllerOnepage
{
    /**
     * Order success action
     *
     * @return MagentoFrameworkControllerResultInterface
     */
    public function execute()
    {
        $session = $this->getOnepage()->getCheckout();
        if (!$this->_objectManager->get('MagentoCheckoutModelSessionSuccessValidator')->isValid()) {
            return $this->resultRedirectFactory->create()->setPath('checkout/cart');
        }
        $session->clearQuote();