using System;

using System.Net;

using System.Text;  // for class Encoding

using System.IO;    // for StreamReader

using Newtonsoft.Json.Linq;

public class Program

{

    public static void Main()

    {

        // created order_id

        String CurrentTimestamp = (DateTime.UtcNow - new DateTime(1970, 1, 1, 0, 0, 0, 0)).TotalSeconds.ToString();

        String order_id = CurrentTimestamp.Replace(".", string.Empty);

        var request = (HttpWebRequest)WebRequest.Create("https://api.upayments.com/test-payment");

        var postData = "merchant_id=" + Uri.EscapeDataString("1201");

            postData += "&username=" + Uri.EscapeDataString("test");

            postData += "&password=" + Uri.EscapeDataString("test");

            postData += "&api_key=" + Uri.EscapeDataString("jtest123");

            postData += "&order_id=" + Uri.EscapeDataString(order_id);

            postData += "&total_price=" + Uri.EscapeDataString("1");

            postData += "&success_url=" + Uri.EscapeDataString("http://example.com/success.html"); // url where you want to redirect user after successful of payment

            postData += "&error_url=" + Uri.EscapeDataString("http://example.com/error.html"); // url where you want to redirect user after failed payment

            postData += "&notifyURL=" + Uri.EscapeDataString("http://example.com/notifyURL.html"); // url we are sending response to this webhook once payment done by payment gateway so you don't need to wait for response

        var data = Encoding.ASCII.GetBytes(postData);

        request.Method = "POST";

        request.ContentType = "application/x-www-form-urlencoded";

        request.ContentLength = data.Length;

        using (var stream = request.GetRequestStream())

        {

            stream.Write(data, 0, data.Length);

        }

        var response = (HttpWebResponse)request.GetResponse();

        var responseString = new StreamReader(response.GetResponseStream()).ReadToEnd();

        JObject jObject = JObject.Parse(responseString); // converting response in JSON

        // here you have to check for error or not

        string paymentURL = jObject["paymentURL"].ToString();

        Console.WriteLine(paymentURL); //it is payment URL.

    }

}