you can't you would have to make it a .aspx page like this.
<%@ Page Language="C#" %><script runat="server">private void Page_Load(object sender, EventArgs e){ ensureHttps();}private void ensureHttps(){ string url = Request.ServerVariables["REMOTE_ADDR"].ToString(); string protocol = url.Substring(0,5); if(String.Compare("https",protocol) != 0) { url = url.Split(':')[1].Replace("//",String.Empty); Response.Redirect("https://" + url); }}</script><html><head> <title></title></head><body></body></html>
If you insist it must be done with a .htm page you will have to use JavaScript, but that is unreliable since users can turn it off. If you still want it I can write you a javascript to do this. Would you like me to?