EDIT global.ascx

<script runat="server">


    void Application_BeginRequest(object sender, EventArgs e)
    {

        if (HttpContext.Current.Request.RawUrl.Contains("/_layouts/mngsiteadmin.aspx"))
        {
            //Response.Redirect("/_layouts/copy_mngsiteadmin.aspx");
            Response.Redirect(HttpContext.Current.Request.RawUrl.Remove(HttpContext.Current.Request.RawUrl.LastIndexOf("/")) + "/custom/HTMLPage1.htm");
        }
    }
  </script>