Ho provato a cambiare completamente il codice, ottengo login fallito.

codice:
'controllo textbox        If TextBox5.Text = Nothing Then
            MessageBox.Show("Please enter your username")
        ElseIf TextBox3.Text = Nothing Then
            MessageBox.Show("Please enter your password")
        Else
            Try
                'Indirizzo URL
                Dim URL As String = "http://mysite/login.php"
                Dim request As HttpWebRequest = WebRequest.Create(URL)
                Dim response As HttpWebResponse = DirectCast(request.GetResponse(), HttpWebResponse)
                Dim reader As StreamReader = New StreamReader(response.GetResponseStream())
                Dim str As String = reader.ReadLine()
                Dim username As String = TextBox5.Text
                Dim password As String = TextBox3.Text




                'Scrivo i dati POST a richiesta
                Dim data As StringBuilder
                request.Method = "POST"
                request.ContentType = "application/x-www-form-urlencoded"




                Dim byteData() As Byte
                Dim postStream As Stream = Nothing




                data = New StringBuilder()
                'Prendo "campo username" variabile contenuta tra virgolette 
                data.Append(Uri.EscapeUriString("username" + TextBox5.Text.ToString))
                'Prendo il "campo password" variabile contenuta tra virgolette
                data.Append(Uri.EscapeUriString("password" + TextBox3.Text.ToString))






                ' Creo un array di byte di dati che vogliamo inviare
                byteData = UTF8Encoding.UTF8.GetBytes(data.ToString())




                'Imposto la lunghezza del contenuto nelle intestazioni di richiesta 
                request.ContentLength = byteData.Length
                response = request.GetResponse()




                'Prendo la risposta flusso in un lettore
                If reader Is New StreamReader(response.GetResponseStream()) Then
                    MessageBox.Show("login successful")
                End If






                response.Close()


            Catch ex As Exception
                MessageBox.Show("login failed")
            End Try
        End If