Meeting rooms have their own calendar in Outlook. Inviting the room to a meeting books it. It accepts automatically if it is free.
New Outlook and Outlook on the web
In the calendar, create a New event or New meeting.
In the location field, click Search for a room or Browse rooms. Available rooms are shown.
Select the room, add attendees and click Send or Save.
Outlook (classic)
Create a New Meeting.
Next to the Location field, click Rooms (or open Room Finder in the ribbon).
Select the room, confirm with OK and Send the invitation.
Tip: Shortly afterwards, you receive a confirmation from the room. If it says “Declined”, the room is already booked.