public PaymentForm Build()
        {
            PaymentForm form = new PaymentForm {
                PostUrl = postUrl
            };

            form.AddField("MerchantNumber", merchantNumber);
            form.AddField("Amount", amount.ToString());
            form.AddField("Currency", currency.ToString());
            form.AddField("OrderID", orderId);
            form.AddField("AcceptURL", acceptUrl);
            form.AddField("DeclineURL", declineUrl);

            if (!string.IsNullOrWhiteSpace(param))
            {
                form.AddField("Param", param);
            }

            if (!string.IsNullOrWhiteSpace(auxParam))
            {
                form.AddField("AuxParam", param);
            }

            if (subscription == 1)
            {
                form.AddField("Subscription", 1.ToString());
            }

            return(form);
        }