(You can user powershell as well, but I dont remember the command in my head and im sitting on a linux laptop 🙂
I suggest that you take a look here as well for Powershell commands to Office 365 Exchange ->
Login into Offce 365 as an administrator. Go into the Administrator pane, and on the left side press “Users” then on the right side, choose the user to want to alter. Then press “More” then “Change Mailbox settings” -> This will redirect you to the Exchange Control Panel. There press E-mail options, and choose add. It will look something like this.
Now this adress will be avaliable to use.