Magento:: add javascript in admin form

>>    $eventElem=$fieldset->addField('nocode', 'checkbox', array(
                'label' => Mage::helper('customer')->__('Event Element'),
                'name'  => 'eventelem',
                'id'    => 'eventelem',
                'value'=>1,
                'onclick'=>'modifyTargetElement(this)',
            ));

        $eventElem->setAfterElementHtml('<script>
            function modifyTargetElement(checkboxElem){
                if(checkboxElem.checked){
                    $("target_element").disabled=true;
                }
                else{
                    $("target_element").disabled=false;
                }
            }
        </script>');
   
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