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Go to Calendar and create a New Appointment
Choose date and time and add a title
Click Location (if you are using an older version of Outlook, you may need to click Scheduling Assistant and then Rooms)
Selecting the Location or Rooms button will open a window with all the configured location.
Select the room you plan to use and click OK.
If you are using Location, be sure to click on Scheduling Assistant tab at the top so you can see if the room is already in use for your selected time. If the room is already in use, the system will not allow you to reserve it for that time.