Estoy intentando iniciar sesión en instagram utilizando solicitudes web. Lo estoy pasando mal entendiendo lo que está pasando. Consiguiendo esto: El servidor remoto devolvió un error: (403) Prohibido. Lo que tengo hasta ahora:

    public static string csrf;
    CookieContainer c1 = new CookieContainer();

    private void button1_Click(object sender, EventArgs e)
    {
        string PostData = String.Format("csrfmiddlewaretoken={0}&username=ra123&password=ra12345678",getToken());

        HttpWebRequest req = (HttpWebRequest)HttpWebRequest.Create("https://instagram.com/accounts/login/");
        req.Method = "POST";
        req.ContentType = "application/x-www-form-urlencoded";
        req.KeepAlive = true;
        req.AllowAutoRedirect = true;              
        req.CookieContainer = c1;
        byte[] byteArray = Encoding.ASCII.GetBytes(PostData);
        req.ContentLength = byteArray.Length;
        Stream dataStream = req.GetRequestStream();
        dataStream.Write(byteArray, 0, byteArray.Length);
        dataStream.Flush();
        dataStream.Close();

            HttpWebResponse webResp = (HttpWebResponse)req.GetResponse();
            Stream datastream = webResp.GetResponseStream();
            StreamReader reader = new StreamReader(datastream);
            string s = reader.ReadToEnd();
            MessageBox.Show(s);
            if (s.Contains("ra123"))
            {

                MessageBox.Show("Loggedin");

            }
            else
            {
                MessageBox.Show("Not");
            }

    }


    string getToken()
    {
        string p = "<input type=\"hidden\" name=\"csrfmiddlewaretoken\" value=\"(.*)\"/>";
        HttpWebRequest req = (HttpWebRequest)HttpWebRequest.Create("https://instagram.com/accounts/login/");
        req.Method = "GET";
        req.CookieContainer = c1;

        HttpWebResponse resp = (HttpWebResponse)req.GetResponse();
        Stream data = resp.GetResponseStream();
        StreamReader sr = new StreamReader(data);
        string src = sr.ReadToEnd();

        Match m = Regex.Match(src, p);
        if (m.Success)
        {
           return (m.Groups[1].Value.ToString());
        }
        return false.ToString();
    }