Dim openScript As String = "<script>" & _
                             " var win; function addGroup(){win =" & _
                             "window.showModalDialog'Child.aspx');" & _
                             " if (win != null){document.getElementById(" & _
                             "'tbReturnVal').value=win;}" & _
                             " </script>"
   Page.RegisterStartupScript("Irgendwas", openScript)
   Me.btnNewGroup.Attributes.Add("onClick", "addGroup();")

If Page.IsPostBack Then
   Dim strReturnVal As String = Request.Form.Item("tbReturnVal").ToString()
end if