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)); }