private static async Task <HttpContent> AddNonce(FormUrlEncodedContent urlEncoded, string strNonce)
        {
            var list = new List <KeyValuePair <string, string> > {
                KeyValuePair.Create("nonce", strNonce)
            };

            NameValueCollection parameters = await urlEncoded.ReadAsFormDataAsync();

            for (int i = 0; i < parameters.Count; i++)
            {
                string[] strings = parameters.GetValues(i);

                string key = parameters.GetKey(i);

                if (strings?.Length != 1)
                {
                    throw new InvalidOperationException($"For key {key} expected only one value");
                }

                list.Add(KeyValuePair.Create(key, strings[0]));
            }

            return(new FormUrlEncodedContent(list));
        }