It is used to specify the id of the form to render. Only required if data-form-url is not used.
Please note: some form IDs start with a minus sign (-), as shown in the example above. Do not delete this as it is a part of the form ID. Note that an advocacy-related form must use the data-form-url attribute instead. Example, to load a non-advocacy-related form with id 12345:
<div class="ngp-form" data-id="12345"></div>