Add the sites that you want to back up to the SharePoint Online app.
Note: To add Project Web App sites, disable Modern Authentication. You can back up Project Web App sites only in a basic authentication environment.
Before You Begin
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.
On the Sites tab, click Add, and then select Add sites.
The Add sites dialog box appears.
From the Office 365 plan list, select the plan to use for the sites.
From the Sites list, select the sites to add.