Add the mailboxes that you want to back up to the Exchange Online app.
You can add a mailbox in the following methods:
If a mailbox is added manually, only the administrator can edit the associated plan and the discovery type for that mailbox to automatic.
You can use the automatic discovery method to automatically find mailboxes for backup and restore operations. The mailbox settings and their plans are inherited from the autodiscovery entity such as the Active Directory group. Any manual modification made to a mailbox plan converts the mailbox type to manual.
To edit the discovery type from manual to automatic for a mailbox, go to Manage > Change plan, and then select the checkbox for Inherit settings from the group configured in the Content tab.
Note: To limit the access of the Azure app to only specific Exchange Online mailboxes, see Limiting application permissions to specific Exchange Online mailboxes.
From the navigation pane, go to Protect > Office 365.
The Office 365 apps page appears.
In the Name column, click the app.
The app page appears.
Click Add, and then click Add Mailbox.
The Add Mailbox dialog box appears.
From the Office 365 plan list, select the Office 365 plan.
From the Select mailboxes list, select the mailboxes to add.