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).