Acceptance checkbox

Hi, you can add fields to form writing some code:
https://www.eventualo.net/blog/easymail-newsletter-for-developers/#tutorial-customfields
https://github.com/groucho75/alo-easymail/blob/master/mu-plugins/alo-easymail_custom-fields.php
For now the only supported “input_type” are “text”, “textarea”, “select”.

It looks like supported fields are needed to write something to database, but the checkbox I’d like to use is a kind of validation thing for the newsletter form. could I edit this form in plugin’s code?