php - How to convert onsubmit to form_open -

my code this:

<form action="http://<?php echo site_url();?>/loader/task1" onsubmit="return checkdifferance();" method="post"> 

how can convert onsubmit form_open? how improve code below?

<?php echo form_open('loder/task1');?> 

try this

<?php echo form_open('loder/task1', array('onsubmit' => 'return checkdifferance();'));?>