Click a link and add a code to my texzt area

[NZ_Kiwis]

I want a code which will add "[20339]" to my text area once I click on it!!!

does someone have a basic code for this?

[esukf]

<script type="text/javascript">
function addtext(txt) {
  	document.form1.textareafield.value = document.form1.textareafield.value + txt;
  	document.form1.textareafield.focus();
  	return true;
 }
</script>
<form name="form1">
<textarea name="textareafield" cols="40" rows="5"></textarea>
</form>

<a href="javascript:void(0)" onclick="addtext('[20339]')">Add [20339]</a>