Office 365, Email, Exchange, Windows, How to grant someone permission to send messages on your behalf
To grant someone permission to send emails on your behalf using Exchange and Outlook, follow these instructions:
How do I give someone the ability to send messages for me with Exchange and Outlook?
Note: If you are using OWA, assigning delegate permissions is not a feature in OWA since you can't open shared calendars as you can in Outlook.
Outlook 2016, 2013