public string HTMLContent(string uri, string postparametters = null)
{
    try
    {
        var httpWebRequest = (HttpWebRequest)WebRequest.Create(uri);
        httpWebRequest.AllowAutoRedirect = false;
        using (var httpWebResponse = (HttpWebResponse)httpWebRequest.GetResponse())
        {
            using (var stream = httpWebResponse.GetResponseStream())
            {
                if (postparametters!= null)
                {
                    System.Text.UTF8Encoding encoding = new System.Text.UTF8Encoding();
                    byte[] postByteArray = encoding.GetBytes(postparametters);
                    stream.Write(postByteArray,0, postByteArray.Length); //!!!Ошибка
                }
                using (var reader = new StreamReader(stream, Encoding.GetEncoding(httpWebResponse.CharacterSet)))
                {
                    return reader.ReadToEnd();
                }
            }
        }
    }

"CountTypeId=1&InGameExchanges=313&InGameIssuerId=185 280&Opener=FORM&UnderlyingType=3&filterBlockClosed=false"