How to enable a free gift voucher based on number of event bookings

Go to Settings > Activities > Events.

Click on the event you wish to enable this feature for. 

Scroll down and click on the Financial green cross symbol. 

You then need to enter information on the following:

Bookings required for gift voucher - enter the number of bookings for this activity that are required per order to receive a free gift voucher.
Free gift voucher amount - enter the amount for the free gift voucher.

Then click Update event.