Aggiungere un campo in un form

[Xinod]

il campo e' presente ma disabilitato e nascosto, x es.

<script type="text/javascript">
function toggleTxt(s){
  var txt=s.form.nomeCampoTesto;
  var bool=(s.options[s.selectedIndex].value=='altro')?1:0;
  txt.style.display=(bool)?'inline':'none';
  txt.disabled=!bool;
}
</script>
<form>
<select onchange="toggleTxt(this)">
<option value="">scegli:</option>
<option value="a">a</option>
<option value="b">b</option>
<option value="altro">...altro...</option>
</select>
<input type="text" name="nomeCampoTesto" disabled="disabled" style="display:none" />
</form>

ciao