This plugin is fully compliant with the General Data Protection Regulation (GDPR).
To enable this checkbox, just go to YITH > Waiting List > Settings and activate the option Enable Privacy checkbox.
When enabled, you will also be able to see a text box, where you can add your custom text to show next to the checkbox. You can also include the shortcode [terms] and [privacy_policy].
Here is an example of how the checkbox appears, right next to the Add to waiting list button.
If you want to enable this checkbox also for logged-in users, you can do that by enabling the Double opt-in option for logged-in users. Please, refer to this page for more details.