I have used https://service.ringcentral.com/faxapi.asp. It raised upgrade your browser error. Couldn't send the fax viia API in C#

I am using the below code. Can you help me what should do change here?

Dictionary<string, object> postParameters = new Dictionary<string, object>();            postParameters.Add("Username", ConfigurationManager.AppSettings["FaxAPIUserName"]);
            postParameters.Add("Password", ConfigurationManager.AppSettings["FaxAPIPassword"]);
            postParameters.Add("Recipient", "1"+BillFax+"");
            postParameters.Add("Coverpage", "0");
            postParameters.Add("Coverpagetext", "JLC Invoice");
            postParameters.Add("Resolution", ConfigurationManager.AppSettings["FaxResolution"]);
            postParameters.Add("Attachment", new FormUpload.FileParameter(data, "invoice.pdf", "application/pdf"));

            // Create request and receive response
            string postURL = ConfigurationManager.AppSettings["FaxAPIUrl"];
            string userAgent = "JLC";
            HttpWebResponse webResponse = FormUpload.MultipartFormDataPost(postURL, userAgent, postParameters);


Please help me regarding this. 

I tried the c# sdk but it cannot support my project. It raised some exception.