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 [1] 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).