HubSpot GoToWebinar Double Post from a form (not a HS landing page)

hubspot_gtw_double_post.aspx

System.Text.StringBuilder GTWoutput = new System.Text.StringBuilder();

//GTW form elements
GTWoutput.Append("WebinarKey=" + Server.UrlEncode(Request.QueryString["WebinarKey"].ToString()));
GTWoutput.Append("&Form=webinarRegistrationForm");
GTWoutput.Append("&Name_First=" + Server.UrlEncode(Request.QueryString["FirstName"].ToString()));
GTWoutput.Append("&Name_Last=" + Server.UrlEncode(Request.QueryString["LastName"].ToString()));
GTWoutput.Append("&Email=" + Server.UrlEncode(Request.QueryString["Email"].ToString()));

//get POST URL for GTW
string GTWurl = "https://www.gotomeeting.com/en_US/island/webinar/registration.flow";

//build submission request 
System.Net.HttpWebRequest GTWreq = (System.Net.HttpWebRequest)System.Net.WebRequest.Create(GTWurl);
GTWreq.Method = "POST"; //must HTTP POST to GTW
GTWreq.ProtocolVersion = System.Net.HttpVersion.Version10; //ASP.Net's implementation of HTTP 1.1 adds a layer that is impossible to work around with GTW's web service
GTWreq.ContentType = "application/x-www-form-urlencoded"; //look like a form submission
GTWreq.KeepAlive = false;

byte[] buffer = Encoding.ASCII.GetBytes(GTWoutput.ToString()); //convert our "form" to a stream-readable format
GTWreq.ContentLength = buffer.Length;

//stream out data over request       
System.IO.Stream PostData = GTWreq.GetRequestStream();
PostData.Write(buffer, 0, buffer.Length); //write to GTW
PostData.Close();

System.Net.HttpWebResponse WebResp = (System.Net.HttpWebResponse)GTWreq.GetResponse(); //get a response from GTW for our request - this CONFIRMS the registration. Miss this step and it won't work.

hubspot_gtw_form.html

<form method="get" action="http://<path-to-file>/gtw-process.aspx">
   First Name: <input name="firstName" size="40" type="text" />
   Last Name: <input name="lastName" size="40" type="text" />
   Email: <input name="email" size="40" type="text" />
   <input name="WebinarKey" type="hidden" value='<ENTER WEBINAR KEY HERE>' />
   <input name="Form" type="hidden" value="webinarRegistrationForm" />
   <input name="Template" type="hidden" value="https://www1.gotomeeting.com/en_US/island/webinar/registration.tmpl" />
   <input type="submit" value="Submit" />
</form>