passing javascript params

I have a search box that I want javascript to open in a new window a google custom search enginne but pass the keyword from my site.

<form name="myform">
<input type="text" name="search_text" id="search_text"> <input type="button" name="search" onclick="goSearch()">
</form>

<script>
function goSearch() {
        var st = document.getElementById('search_text').value;
        window.open('https://www.google.com/cse/publicurl?cx=016143464297381313748:e13kxohrq8s' + st);
}
</script>

Firebug indicates the element ID is null. How do I capture the data for "search_text" with the onclick?