how to get request["PROD_ID"] from HPP post response from realex payment

Question

I have done a post using controller to the RealexPaymentRedirectRequest i use controller redirect request post to realex payments.

RedirectRequest postData = new RealexPaymentRedirectRequest()
                                        {
                                            Account = Sitecore.Configuration.Settings.GetSetting("realexaccount", "internet"),
                                            AutoSettleFlag = "1",
                                            Currency = "GBP",
                                            MerchantId = Sitecore.Configuration.Settings.GetSetting("realexmerchant", "merchant"),
                                            MerchantResponseUrl = Sitecore.Configuration.Settings.GetSetting("realexmerchantResponseUrl", "http://www.mydomainhost.uk/service/CompletePayment.ashx"),
                                            OrderId = orderId,
                                            Timestamp = RealexPaymentRedirectRequest.GenerateTimestamp(),
                                            Secret = Sitecore.Configuration.Settings.GetSetting("realexsecret", "topsceret"),

                                            Vat = vatRate != 0 ? vatRate : Sitecore.Configuration.Settings.GetIntSetting("realexdefaultVatRate", 20),
                                            Amount = amountToPay,
                                            ProdId = prodId
                                        };

I would like to get the prod id string back to my confirmation page. I have tried from HPP response request using request[PROD_ID], but this is returning empty string.