Office 365 Outlook Web App - Calendar Free/Busy
How to set free/busy time in Office 365 Calendar
In Calendar, click on New.
Under Details, type a title and location (optional).
Fill in Start date and time.
Next fill in End date and time and check Private, if needed.
Fill in the fields Repeat, From, To, Reminder and Show As.
Add to the body section if needed and then click Save and close.