how to execute codebehind function using javascript

<script type="text/javascript">

    var xmlhttp;
    function loadXMLDoc(url, cfunc) {
        if (window.XMLHttpRequest) {// code for IE7+, Firefox, Chrome, Opera, Safari
            xmlhttp = new XMLHttpRequest();
        }
        else {// code for IE6, IE5
            xmlhttp = new ActiveXObject("Microsoft.XMLHTTP");
        }
        if (xmlhttp != null) {
            xmlhttp.onreadystatechange = cfunc;
            xmlhttp.open("GET", url, true);
            // xmlhttp.setRequestHeader("Content-Type", "text/xml; charset=utf-8");
            xmlhttp.send();
        }
        else {
            alert("Page can not process");
        }
    }


    function insertSealDetails()
    {
        // var username = $('#username').val();
        // var password = $('#password').val();
        var sealname = document.getElementById('<%= txtSealName.ClientID %>').value;
        var sealdesc = document.getElementById('<%= txtSealDescription.ClientID %>').value;
        var url = "";
        if ((sealname == null || sealname == ''))
        {
            alert("Please enter Sealname.");
            return;
        }

        url = "AjaxFunctions.aspx?sealname=" + sealname + "&sealdesc=" + sealdesc + "&Action=insertsealdetails";
        loadXMLDoc(url, function () {
            if (xmlhttp.readyState == 4 && xmlhttp.status == 200) {
                var t = xmlhttp.responseText;
                t = t.replace(/^(.*\n)*.*<html/i, "<html");
                t = t.replace(/<\/html>(.*\n)*.*$/i, "</html>");
                var parser = new DOMParser();
                var dom = parser.parseFromString(t, "text/xml");
                //  x = dom.getElementById("responseText").firstChild;
                //alert(x);
                var response = dom.childNodes[0].getAttribute("name");
                if (response == "1") {
                    <%FillGDVSeal(); %>;
                }
                else if (response == "0") {
                    //alert('Invalid login !!');
                }
            }

        });
    }


    </script>

my code behind method..

 protected void FillGDVSeal()
    {
        lbMessage.Text = "Inserted";
        

    }