¿Cómo se debe modificar el siguiente código para enviar elWebRequest a través de un especificadoproxy server yport number?

Dim Request As HttpWebRequest = WebRequest.Create(url)
Request.Method = "POST"
Request.ContentType = "application/x-www-form-urlencoded"
Using writer As StreamWriter = New StreamWriter(Request.GetRequestStream())
    writer.Write(params)
End Using