where is admin footer?

So the question that you found this code from here says that they added this to the admin_footer action. So you could add that custom_bulk_admin_footer to your functions.php, and then hook it into the admin_footer action with this after the function:

add_action('admin_footer', 'custom_bulk_admin_footer');

Obviously you need to change the jQuery code to do what you want inside the function too 😉

HTH