Can I set that a class/tour/game/event/workshop can be booked only by customers with a specific/secret link?
You can hide a class/tour/game/event/workshop on a specific date and time from your booking page, and set that only customers with a specific link can make bookings for it.
1. Go to your Calendar
2. Click on the class/tour/game/event/workshop on the specific date and time
3. Go to the Access tab
4. Tick the Hidden  checkbox
5. Right click with your mouse on the link, select Copy link address
6. Click on the Save button
7. Give the copied link to your customer(s).
Note: you can also change the visibility of the type of class/tour/game/event, to "only I and authorized users". See http://help.bookeo.com/customer/en/portal/articles/113057-tours-and-activities-visibility-any-customers-members-only-show-to-non-members-bookeo-users-only-hide-from-customers
In this case, all events are hidden by default, so you do not need to tick the "Hidden" checkbox. But the link will still be valid, and can still be used by customers.