How do I use C# to post to this form grammatically?

I am trying to check my gift card balance, (I have many) on the following website and I want to automate via a C# program.

http://ift.tt/2buoqOF

It is form1 in the source code im interested in. This is my code:

class Program
{


    public static string HttpPost(string url, params object[] postData)
    {
        StringBuilder post = new StringBuilder();
        for (int i = 0; i < postData.Length; i += 2)
            post.Append(string.Format("{0}{1}={2}", i == 0 ? "" : "&", postData[i], postData[i + 1]));
        return HttpPost(url, post.ToString());
    }
    public static string HttpPost(string url, string postData)
    {
        postData = postData.Replace("\r\n", "");
        try
        {
            WebRequest req = WebRequest.Create(url);
            byte[] send = Encoding.Default.GetBytes(postData);
            req.Method = "POST";
            req.ContentType = "application/x-www-form-urlencoded";
            req.ContentLength = send.Length;

            Stream sout = req.GetRequestStream();
            sout.Write(send, 0, send.Length);
            sout.Flush();
            sout.Close();

            WebResponse res = req.GetResponse();
            StreamReader sr = new StreamReader(res.GetResponseStream());
            string returnvalue = sr.ReadToEnd();
            return returnvalue;
        }
        catch (Exception ex)
        {
            Debug.WriteLine("POST Error on {0}\n  {1}", url, ex.Message);
            return "";
        }


    }

    static void Main(string[] args)
    {


        HttpPost("http://ift.tt/2j3cupz", 
            "CardNumber", "CREDIT NUMBER",
            "ExpirationMonth", "MONTH",
            "ExpirationYear", "YEAR",
            "SecurityCode", "CODE");
    }
}

I just want to see if the website would give me a response. The following error is what I get:

POST Error on http://ift.tt/2j3cupz
  The remote server returned an error: (404) Not Found.

How do I figure out what the real URL should be?