How do I get a form element to display in a lightbox?
If you want to display form element into popup, then I would suggest you to use ctools api. It will provide you functionality to create a custom popup to render content.


  Ctools provide Modal dialog functionality to display element in modal.


You need to follow below step for the same:


Create a custom module and create Menu item which will interact with ctools modal.
Define ajax callback for the menu item and return HTML of the modal
using ajax.

You can check working example  (http://deeson-online.co.uk/labs/insert-form-pop-modal-ctools-and-drupal-7)