HttpWebRequest failed to login onto web site

Imports System.Net
Imports System.Text
Imports System.IO

Public Class Form1

    Dim logincookie As CookieContainer

    Private Sub Button1_Click(sender As Object, e As EventArgs) Handles Button1.Click

        Dim postData As String = "auth_key=880ea6a14ea49e853634fbdc5015a024&rememberMe=1&referer=http%3A%2F%2Fforums.zybez.net%2F&ips_username=" & TextBox1.Text & "&ips_password=" & TextBox2.Text & "&submit=Login"
        Dim tempCookies As New CookieContainer
        Dim encoding As New UTF8Encoding
        Dim byteData As Byte() = encoding.GetBytes(postData)

        Dim postReq As HttpWebRequest = DirectCast(WebRequest.Create("https://forums.zybez.net/index.php?app=curseauth&module=global&section=login&do=process"), HttpWebRequest)
        postReq.Method = "POST"
        postReq.KeepAlive = True
        postReq.CookieContainer = tempCookies
        postReq.ContentType = "application/x-www-form-urlencoded"
        postReq.Referer = "https://forums.zybez.net/index.php?app=curseauth&module=global&section=login&do=process"
        postReq.UserAgent = "Mozilla/5.0 (Windows NT 6.1; Win64; x64; rv:47.0) Gecko/20100101 Firefox/47.0"
        postReq.ContentLength = byteData.Length
        'ServicePointManager.SecurityProtocol = SecurityProtocolType.Ssl3


        Dim postreqstream As Stream = postReq.GetRequestStream()
        postreqstream.Write(byteData, 0, byteData.Length)
        postreqstream.Close()

        Dim postresponse As HttpWebResponse
        postresponse = DirectCast(postReq.GetResponse(), HttpWebResponse)
        tempCookies.Add(postresponse.Cookies)
        logincookie = tempCookies

        Dim postreqreader As New StreamReader(postresponse.GetResponseStream())

        Dim thepage As String = postreqreader.ReadToEnd

        RichTextBox1.Text = thepage

    End Sub

    Private Sub Button2_Click(sender As Object, e As EventArgs) Handles Button2.Click

        WebBrowser1.DocumentText = RichTextBox1.Text

    End Sub

End Class

Friend Function GetBetween(ByVal haystack As String, ByVal needle As String, ByVal needle_two As String) As String
        Dim istart As Integer = InStr(haystack, needle)
        If istart > 0 Then
            Dim istop As Integer = InStr(istart + Len(needle), haystack, needle_two)
            If istop > 0 Then
                Try
                    Dim value As String = haystack.Substring(istart + Len(needle) - 1, istop - istart - Len(needle))
                    Return value
                Catch ex As Exception
                    Return ""
                End Try
            End If
        End If
        Return ""
    End Function