After you install the SQL agent on all the physical nodes of an Always On cluster, create an availability group instance.
From the navigation pane, go to Protect > Databases.
The Instances page appears.
Click Add Instance, and then click SQL availability group.
The Add SQL availability group instance dialog box appears.
From the Primary replica list, select the SQL Server client that is the primary replica of the availability group that you want to back up.
From the SQL Server instance list, select an instance.
If the SQL Server client has only one instance, then the instance is automatically selected.
For Accounts, select one of the following options:
To use a local system user account for the backups, click Use local system account.
To use a different Windows user account, click Impersonate User, and then enter the credentials for the account.
Note: The account must be set up on the client and must have Local administrator privileges and be a member of the SQL sysadmin fixed server role for the instance.
From the Plan list, select the server plan to use for the instance.