Silverstripe 3 - add css class to CMS form field

Question

How can I go about add a css class to the CheckboxSetField below - I've tried the setAttribute method but it has no effect (using SS 3.0):

$checkboxFieldSet = new CheckboxSetField('CampusPage', 'Campuses (select multiple)*', $campusArea->map());

Answer 1

$checkboxFieldSet->addExtraClass("my-class");

API Docs: http://api.silverstripe.org/3.1/class-FormField.html#_addExtraClass