<script type="text/javascript">
                function chkChange(ddl)
                {
                    var textbox = document.getElementById("<%= txtdept.ClientID %>");
 
                    if (ddl.options[ddl.selectedIndex].value == "New Department")
                {
                    textbox.style.display = "block";
                }
                    else
                {
                    textbox.style.display = "none";
                }
document.getElementById("<%= ddldept.ClientID %>").options.add(new Option(index, "value", "text"));
                }
                </script>

 <script type="text/javascript">
    function chkChange(ddl)
    {
        var textbox = document.getElementById("<%= txtdept.ClientID %>");
 
        if (ddl.options[ddl.selectedIndex].value == "New Department")
        {
            elm.onblur = function() { addOptionOnBlur(this); };
            textbox.style.display = "block";
            textbox.focus();
        }
        else
        {
            textbox.style.display = "none";
        }
    }
 
    function addOptionOnBlur(elm)
    {
        if (elm.value.length > 0 && elm.value.replace(/ /g, "").length > 0)
        {
            document.getElementById("<%= ddldept.ClientID %>").options.add(newOption(elm.value, elm.value));
        }
    }
 
    function newOption(val, txt)
    {
        var option = document.createElement("OPTION");
        option.text = txt;
        option.value = val;
 
        return option;
    }
</script>