come da esempio :

in head:
codice:
<script type="text/javascript" language="JavaScript">
function leapto(form) {

	var myindex=form.dest.selectedIndex

	window.open(form.dest.options[myindex].value);
}
</script>

in body:
codice:
<form name="myform">
	<select name="dest" SIZE=1>
		<option SELECTED value="">--->scegli una voce<---</option>
		<option value="http://www.html.it">HTML.it</option>
		<option value="http://freephp.html.it">FREEPHP</option>
		<option value="http://freeasp.html.it">FREEASP</option>
		<option value="http://forum.html.it">FORUM</option>
	</select>
	<input type="button" value="Vai!" onClick="leapto(this.form)">
</form>