getting confused in adding checkout custom field

-- url of the page with the problem -- : developement site
-- HikaShop version -- : 2.2.3
-- Joomla version -- : 3.2.3
-- PHP version -- : 5.3.27

I am getting confused in adding custom field of single dropdown in checkout form(front end). Actually I got the way of adding custom fields from joomla admin. But my dropdown is gonna be dynamic. It would have dynamic values in its options. So how can I add that options values in it ? Please suggest !

Thanks .