client side authorization in asp.net web project

i am developing a server-client program in which server will be showing datas sent by client across a network. without Authentication it works fine but i need some authentication because server side its a website.once i give authentication i am not able to get datas from client.i know that client must be given authorization to put data into server.can anyone tell me how to do that?i am developing based on the link given below

blog.bobcravens.com/2009/11/browse-files-on-remote-computers

posting server side authentication code

<authentication mode="Forms">
  <forms loginUrl="Login.aspx" name=".ASPXFORMSAUTH">


  </forms>
</authentication>

<authorization>
  <deny users = "?"/>
</authorization>

login.aspx

<script runat="server">
    Sub Login_Click(ByVal sender As Object, ByVal e As EventArgs)
        If ((UserEmail.Text = "s@gmail.com") And _
            (UserPass.Text = "000")) Then
            FormsAuthentication.RedirectFromLoginPage _
                 (UserEmail.Text, Persist.Checked)
        Else
            Msg.Text = "Invalid credentials. Please try again."
        End If
    End Sub
</script>

i have written validation code also.everything works fine on the server side but not able to get data from client side..

client program is written in c#,client-server communication is via http post.

i am getting error shown below when i run client program