Using ASP.net to pass parameters to external website - asp.net

Question

I am trying to access a page where I need to log into the website and pass a a list of parameters. I appear to be able to log into the site (if I change the log in details I get a 401 unauthorised error) but then I get a 400 bad request error. The code is a bit hashed together so I know something is wrong but don't know where.
EDITED CODE
Public Sub TestConn()
Dim customeremailaddress As String = HttpUtility.UrlEncode("r.test#test.com")
Dim customername As String = HttpUtility.UrlEncode("Ryan")
Dim referenceid As String = HttpUtility.UrlEncode("ordertest123")
Dim languagecode As String = HttpUtility.UrlEncode("1043")
Dim expirydays As String = HttpUtility.UrlEncode("30")
Dim UserName As String = "testusername"
Dim password As String = "testpassword"
Dim siteCredentials As New NetworkCredential(UserName, password)
Dim URLAuth As String = "http://service.someurl.com/process.xml"
Dim postString As String = String.Format("customeremailaddress={0}&customername={1}&referenceid={2}&languagecode={3}&expirydays={4}", customeremailaddress, customername, referenceid, languagecode, expirydays)
Dim postBytes As Byte() = Encoding.UTF8.GetBytes(postString)
Const contentType As String = "application/x-www-form-urlencoded"
System.Net.ServicePointManager.Expect100Continue = False
Dim cookies As New CookieContainer()
Dim webRequest__1 As HttpWebRequest = TryCast(WebRequest.Create(URLAuth), HttpWebRequest)
webRequest__1.Method = "POST"
webRequest__1.ContentType = contentType
webRequest__1.CookieContainer = cookies
webRequest__1.ContentLength = postBytes.Length
webRequest__1.UserAgent = "Mozilla/5.0 (Windows; U; Windows NT 5.1; en-US; rv:1.9.0.1) Gecko/2008070208 Firefox/3.0.1"
webRequest__1.Accept = "text/html,application/xhtml+xml,application/xml;q=0.9,*/*;q=0.8"
webRequest__1.Referer = "http://service.someurl.com/process.xml"
webRequest__1.Credentials = siteCredentials
Try
Dim requestStream As Stream = webRequest__1.GetRequestStream()
requestStream.Write(postBytes, 0, postBytes.Length)
Dim responseReader As New StreamReader(webRequest__1.GetResponse().GetResponseStream())
Dim responseData As String = responseReader.ReadToEnd()
responseReader.Close()
webRequest__1.GetResponse().Close()
Catch ex As Exception
Lbl_ConnTest_error.Text = ex.Message
End Try
End Sub

Answer 1

You need to send the bytes of the postString not the string itself:
Dim postBytes As Byte() = Encoding.UTF8.GetBytes(postString)
...
webRequest__1.ContentLength = postBytes.Length
...
Dim requestStream As Stream = webRequest__1.GetRequestStream()
requestStream.Write(postBytes, 0, postBytes.Length)
More information here.