How to add Calendars to my OWA

Switch to the Calendar tab on the left menu.

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By default there would be just your Calendar under "My calendars" list. 
By clicking on "Add calendar" you can add shared ones which are representing Boardrooms across our offices.
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Continu to "Add from directory" in new window. On the top in the section call "Please select an account to search from:" select your email address.
Then in the search bar bellow you would be able to search and add new calendar to your OWA.

How to book an event in a shared calendar

Click on a new Event
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New window will appear now you can enter details about your event, invite attendees and select desired Boardroom
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