Hello.
I can suggest that you need to set some fields as required deoend from selected options in checkboxes or selectboxes in your booking form, if so then unfortunately, by default this functionality does not available in original versions of Booking Calendar.
You will be need to create some custom JavaScript code, which will add or remove these CSS classes “wpdev-validates-as-required” to the text fields, for checking these fields as required fields, during submitting of the booking form.
Disclaimer. Unfortunately we can not start, right now, some personal customization or custom development, because almost have no free time.
Check more about new features here https://wpbookingcalendar.com/faq/need-new-feature/
Thank you for understanding.
Please note, if you will modify the source code of the Booking Calendar, we will not guaranteed the correct work of plugin and do not support it.
Kind Regards.
