How to use input hidden fields in smarty form

I want to use hidden field in smarty form

tell me the how i use this

i know how to add element like this

$form -> addElement('submit', 'submit_order', Pay, 'class = "flatButton"');

but i want to used hidden fields with this

Have you tried

$form->addElement('hidden', 'fieldname', 'Text', '');

?