1. Open Outlook WebApp Calendar
  2. Click on New event button
  3. Fill the meeting title, attendees and select a room. You might need to click on Add room if the room you are looking for is not listed.
  4. All the meeting rooms will be displayed on the right side "Browse Rooms" pane. If you still cannot find the meeting room you are looking for, please request its creation to IT via an Helpdesk ticket.
  5. Save/Send your meeting for the room to be effectively booked.

More info Outlook WebApp - Outlook Desktop