Every event type you have is accessible via direct link.
By default every event type is shown in your main FreeBusy offering page. If you want to unlist it:
Go to Share page
Click Configure for your event type
Set Unlisted under your image in first section and Save
You can pick a vanity link that’s hard to guess, by generating a random 10 character string which effectively acts as an implicit password known only to those to whom you send the link.