These steps are similar both in the Outlook Desktop App, and the Office 365 Outlook Web App Online
\uD83D\uDCD8 Instructions
Using the Scheduling Assistant
From the Inbox, select New Items > Meeting or in the Calendar, select New Meeting. You also can use the keyboard shortcut Ctrl+Shift+Q to create a new meeting request.
In a new meeting, add the Required attendees, the Start time and the End time, then click the Location button.
Select the boardroom you wish to book, then select Ok.
The boardroom will automatically accept the invite and the room will be booked for your time slot.
For more information on how to connect to the boardroom display, see this article.