public override void RequestModify(HttpRequestMessage request, CancellationToken cancellationToken)
{
var headers = request.Headers;
var prevData = ApiHelpers.DecodeUrlEncodedBody(request.Content.ReadAsStringAsync().Result).ToList();
var postData = new List <KeyValuePair <string, string> >();
postData.Add(new KeyValuePair <string, string>("nonce", GetNonce().ToString()));
postData.AddRange(prevData);
var bodyDataEnc = postData.Select(x => $"{x.Key}={x.Value}").ToArray();
var message = string.Join("&", bodyDataEnc);
var sign = HashHMACSHA512Hex(message, ApiKey.Secret);
request.Content = new FormUrlEncodedContent(postData);
headers.Add("KEY", ApiKey.Key);
headers.Add("Sign", sign);
}