如何更改此代码以处理定期付款?

我正试图实现直接支付方式，因为我在我的数据库中有用户的信用卡信息等。这就是我所指的示例:

using System;
using System.Collections.Generic;
using System.Text;
using com.paypal.sdk.services;
using com.paypal.sdk.profiles;
using com.paypal.sdk.util;
using com.paypal.soap.api;
namespace ASPDotNetSamples
{
    public class DoDirectPayment
    {
        public DoDirectPayment()
        {
        }
        public string DoDirectPaymentCode(string paymentAction, string amount, string creditCardType, string creditCardNumber, string expdate_month, string cvv2Number, string firstName, string lastName, string address1, string city, string state, string zip, string countryCode, string currencyCode)
        {
            com.paypal.soap.api.DoDirectPaymentReq req = new com.paypal.soap.api.DoDirectPaymentReq();

            NVPCallerServices caller = new NVPCallerServices();
            IAPIProfile profile = ProfileFactory.createSignatureAPIProfile();
            /*
             WARNING: Do not embed plaintext credentials in your application code.
             Doing so is insecure and against best practices.
             Your API credentials must be handled securely. Please consider
             encrypting them for use in any production environment, and ensure
             that only authorized individuals may view or modify them.
             */
            // Set up your API credentials, PayPal end point, API operation and version.
            profile.APIUsername = "sdk-three_api1.sdk.com";
            profile.APIPassword = "QFZCWN5HZM8VBG7Q";
            profile.APISignature = "AVGidzoSQiGWu.lGj3z15HLczXaaAcK6imHawrjefqgclVwBe8imgCHZ";
            profile.Environment = "sandbox";
            caller.APIProfile = profile;
            NVPCodec encoder = new NVPCodec();
            encoder["VERSION"] = "51.0";
            encoder["METHOD"] = "DoDirectPayment";
            // Add request-specific fields to the request.
            encoder["PAYMENTACTION"] = paymentAction;
            encoder["AMT"] = amount;
            encoder["CREDITCARDTYPE"] = creditCardType;
            encoder["ACCT"] = creditCardNumber;
            encoder["EXPDATE"] = expdate_month;
            encoder["CVV2"] = cvv2Number;
            encoder["FIRSTNAME"] = firstName;
            encoder["LASTNAME"] = lastName;
            encoder["STREET"] = address1;
            encoder["CITY"] = city;
            encoder["STATE"] = state;
            encoder["ZIP"] = zip;
            encoder["COUNTRYCODE"] = countryCode;
            encoder["CURRENCYCODE"] = currencyCode;
            // Execute the API operation and obtain the response.
            string pStrrequestforNvp = encoder.Encode();
            string pStresponsenvp = caller.Call(pStrrequestforNvp);
            NVPCodec decoder = new NVPCodec();
            decoder.Decode(pStresponsenvp);
            return decoder["ACK"];
        }
    }
}

这是链接:

https://cms.paypal.com/cms_content/US/en_US/files/developer/nvp_DoDirectPayment_cs.txt

这工作得很好，但我唯一的问题是我如何处理与此定期付款?我应该对样品做哪些修改?

提前感谢:)