The free time / busy option in Microsoft Exchange is created from Outlook / Exchange Calendar entries from users. This information is retrieved from the users calendar by the accessibility service, as Microsoft writes here . Therefore, if you want something "SET", you need to create a calendar entry for the user. If you try to add something to the back-end environment managed by Microsoft Exchange, you can create problems for users because they do not see what is on the calendars. This is also the reason that you cannot find the "API for writing free time / busy time". Therefore, please create a calendar entry for your purpose and let the MS Exchange Availability Service do the rest.
A good starting point for understanding the design is: