C#调用webapi服务post请求带请求头

/// 

        /// post请求
        /// 
        /// 请求内容
        /// 请求地址
        /// 
        public string HttpPost(string body,string urlStr)
        {
            string retString = string.Empty;
            string paramData = body;
            HttpWebRequest request = (HttpWebRequest)WebRequest.Create(urlStr);
            request.Method = "POST";
            byte[] bytes = Encoding.UTF8.GetBytes(paramData);
            request.Accept = "*/*";
            request.ContentType = "application/json;charset=UTF-8";

            string hsf_timestamp = DateTimeUtil.DateTimeToTimeStamp(DateTime.Now).ToString();//时间戳
            string infosyssign = serv_sign;
            string hsf_nonce = Guid.NewGuid().ToString("N");
            string hsf_si