_global.asax

This is a book about vb.you could learn this from this book

<%@ Import Namespace="Microsoft.WebServices.Security" %>
<%@ Import Namespace="System.Security.Principal" %>

<script Language="VB" runat="server">
  Public Sub WebServiceAuthentication_OnAuthenticate(sender As Object, e As WebServiceAuthenticationEvent)
    If ((e.User = "[e-mail]") AND (e.Password = "LetMeIn")) Then
'      e.Authenticate()

      Dim s(1) As String
      s(0) = "Customer"
      e.Authenticate(s)
    End If
  End Sub
</script>