拼多多需要服务商自己实现加签接口，只需要data_to_sign，对data_to_sign字段中的值实现加签

string datasign = JsonConvert.SerializeObject(request["data_to_sign"]);

string sgin = SignHelper.SignatureValue(JsonConvert.SerializeObject(request["data_to_sign"])).Item1;

                jObject["response_code"] = 1;
                jObject["response_msg"] = "加签成功。";
                jObject["signature"] = sgin;

return jObject;

public static Tuple<string, string> SignatureValue(string xmlValue, bool isKeyInfo = false)
        {
            State state = WebSocketSign(xmlValue);
            JObject jObject = JObject.Parse(state.Data);
            JArray jArray = jObject["Data"].ToObject<JArray>();

            string keyName = null;

            if (isKeyInfo)
                keyName = jArray[1].ToString();

            return Tuple.Create(jArray[0].ToString(), keyName);
        }

具体加签实现：记：.net 海关179自行加签方法_q1606671375的博客-CSDN博客