用 HttpWebRequest 來抓取資料 ，以下程式碼試試看傳值可以用 ?objid=a1234 一樣可以導回資料

string _strReturnVaues = string.Empty;
if (_strContentTYpe == null || _strContentTYpe.Length == 0)
{
    _strContentTYpe = "text/html";
}

try
{
    HttpWebRequest _HttpWebRequest = (HttpWebRequest)WebRequest.Create(_strWebSite + _strWebFilesNAME);
    _HttpWebRequest.Method = WebRequestMethods.Http.Get;
    _HttpWebRequest.ContentType = _strContentTYpe;  //"application/json";

    using (var _UsingResponse = (HttpWebResponse)_HttpWebRequest.GetResponse())
    {
        if (_UsingResponse.StatusCode == HttpStatusCode.OK)
        {
            using (var _varStream = _UsingResponse.GetResponseStream())
            {
                using (var _varReader = new StreamReader(_varStream))
                {
                    _strReturnVaues = _varReader.ReadToEnd();
                }
            }
        }
        else
        {
            _ReadErrorMessage = "讀取失敗：" + _UsingResponse.StatusCode.ToString();
            _ReadErrorMessage += "請確定您要讀取的 " + _strWebSite + _strWebFilesNAME + " 是否有存在!!";
        }
    }
}
catch (Exception onerror)
{
    _ReadErrorMessage = onerror.Message;
    //指定為空白的內容
    _strReturnVaues = "";
}

用WebClient+POST

using (WebClient wc = new WebClient())
{
  try
  {
      wc.Encoding = Encoding.UTF8;
      
      NameValueCollection nc = new NameValueCollection();
      nc["id"] = "aaa";
      nc["pw"] = "bbb";
      
      byte[] bResult = wc.UploadValues(Config.PostURL, nc);
      
      string resultXML = Encoding.UTF8.GetString(bResult);
  }
  catch (WebException ex)
  {
      throw new Exception("無法連接遠端伺服器");
  }
}

引用出處
https://blog.miniasp.com/post/2010/01/23/Emulate-Form-POST-with-WebClient-class