string ch1 = ResolveClientUrl("GestionEngagement.aspx");

string ch2 = @" popup.onunload=function()
                {
                    var test = $.getJSON('../AjaxTraitementOnUnload.aspx', {'IdTraitement' : 1}, function() { });
                };";

string script = "<script language=JavaScript>var popup = window.open('" + ch1 + "', '', 'width=1034, height=600, resizable=yes, location=no, menubar=no, status=no, scrollbars=yes '); " + ch2 + " </script>";
this.Page.ClientScript.RegisterStartupScript(this.GetType(), "startup", script);