FreeBusy works directly with your calendar when working out your availability.
It will show you busy whenever there's an event on your calendar. Whenever you're not busy in your calendar, FreeBusy will show you as available.

If the times when you're available are in the minority, you can take advantage of our Guarded Availability feature.

If you don't want to expose your calendar availability and want to manually add #free time-range in you calendar (and automatically show the rest of your time as busy), you can do this by selecting Guarded Availability. Configure an event template, go to the section "When are you generally available for this meeting?", and choose "Guarded `Availability" in the "Availability control" section.

By adding #free time-range in your calendar, FreeBusy will show you available for meetings only in specified free slots.

Learn how to add free time slots in your calendar, in this article (the same in Outlook) How to create "Free" or "Available" events using Google Calendar.

If you have any questions & feedback, feel free to use the chat button below. We're super friendly and happy to solve any scheduling challenge.