an_action.ctp

<?php
	echo $this->Form->create('Event');
	echo $this->Form->input('Event.reservation_id', array('empty' => __('Create a reservation for this event'), 'required' => false));
	echo $this->Form->end();

	$this->Js->buffer('urls.reservationsGet = "' . $this->Html->url(array('controller' => 'reservations', 'action' => 'get')) . '/";');
?>

<script type="text/javascript">
$('#EventReservationId').on('change', function() {
	$.ajax(urls.reservationsGet + $(this).val());
});
</script>