I have came across some interesting scenario where Exchange Server doesn’t exist however some attributes might be still required or used on Office 365 for Exchange online users which are Synced with Azure Active Directory Sync tool.
The attributes might be used for different purposes but sometimes it’s very necessary so I will go ahead and demonstrate how to create a custom attribute which is normally created by default with Exchange servers deployed on-prem.
First I will run the Schema console
On one of the DC servers which are synced with Office 365 Launch CMD as Administrator
Run the following Command
Click on File -> ADD /Remove Snap in
Right Click on Attributes -> Create Attribute…
Click on Continue
Go to Attributes, Navigate to Custom attributes and double click on it and tick the boxes below then click apply
Now go to Classes
Find and double click on User
Click on Add and add the Custom Attribute
Now go back to CMD on DC Server and replicate changes across all servers
Open ADUC and check users attributes
Hope this helps