Adding fields in the form

Hello.
If you want to automatically check (if its true), that this user is your member, so its does not possible. Sorry.
But you can configure any checkboxes or selectboxes in your booking form at the Booking > Settings > Form page.

Please note, if you need to define additional cost for the booking depend from the selected options in the checkboxes or selectboxes in your booking form, so then its possible in the Booking Calendar Business Medium or higher versions. Check more about this feature here: https://wpbookingcalendar.com/overview/#additional-cost
Watch it in this video guide https://wpbookingcalendar.com/help/advanced-costs-video-overview/
You can test Booking Calendar live demos at this page https://wpbookingcalendar.com/demo/ to be sure in functionality and do not have misunderstanding.
Kind Regards.