To send a private event to your customers, you'll have create a new service. Head to your business > services > create new service. Make sure to set this service as "Not visible online"
Now you'll have to create the widget so that you're able to get a private URL.
Go to booking tools > booking widgets > create new widget > give it a name > filter by service > choose the new service you created > save widget > get the widget URL.
This service won't show on your microsite or other booking tools and will only be available to people with the link😊