new select box option

<select onChange="checkNew(this);" name="selectName">
<option value="0">Add new...</option>
</select>
 
    <script LANGUAGE="JavaScript" TYPE="text/javascript">
        function checkNew(frm) {
 
            var selectedOption = frm.selectedIndex;
            var selectedValue = frm.options[selectedOption].value;
            var selectedText = frm.options[selectedOption].text;
 
            if (selectedValue == '0') {
                var newOption = prompt('Enter new ','');
                frm.options[frm.options.length] = new Option(newOption,newOption,true); 
            }
        }
    </script>