<script type="text/javascript">
function getSelId(obj){
document.getElementById('textfield').value = obj.options[obj.selectedIndex].value;}
</script>
<p>
<select name="select" onchange="getSelId(this)">
<option value="1" selected>1</option>
<option value="2">2</option>
<option value="3">3</option>
</select>
</p>
<p>
<input type="text" name="textfield">
</p>