вы можете сделать так:
<?php
echo $this->Form->label('short_link', 'Custom URL <span style="font-size:9px;"><em>(Optional)</em></span>:', array('class' => 'YOUR CLASS FOR LABEL'));
echo $this->Form->input('short_link',array('label'=>false,'class'=>'YOUR CLASS FOR INPUT','value'=>'','size'=>'40'));
?>