Enter keypress function in Form doesn't work

Question

I did this form to calculate shippings:

<div class="container-frete">
<div class="title-frete">
    Simulação de Entrega:<br>
   <div class="label-frete"> Digite seu CEP<br>  </div>
    <input id="cep" type="text"/>
    <input id="botao" type="button" class="button-frete" value="<?php echo $this->__('Calcular'); ?>"/>
</div>
<div id="content-frete">

</div>

But I don't know how to make enter keypress work on it:

<script>

  Event.observe('botao', 'click', function(event) {
      new Ajax.Updater('content-frete', '/Freteproduto?cep='+$('cep').getValue()+'&productid=<?php echo $produto->getId();?>', { method: 'get' });

  });

</script>

Answer 1

I Make a new one in jquery

    jQuery.noConflict(); // INSIDE INPUT "CEP" 
if (jQuery('input#cep').length) { 
    jQuery('input#cep').click(function(){ // GET KEYPRESS "E"
        jQuery(document).keypress(function(e) { 
            var keycode = (e.keyCode ? e.keyCode : e.which); // ENTER? 
            if(keycode == '13') { 
                new Ajax.Updater('content-frete', '/Freteproduto?cep='+$('cep').getValue()+'&productid=<?php echo $produto->getId();?>', { method: 'get' });
            } 
        }); 
    }); 
}