Connect to the session as an appropriate admin user (this can be done using either Windows or SQL authentication) to the SQL instance
Expand Security->Logins
Right-click Logins and choose “new Login”
Add the backup service account details, ensuring that you select SQL authentication and deselect “enforce password policy”
Select “User mapping” on the left-hand panel tab, and ensure that ” db_backupoperator is ticked for the databases you wish to backup – you do not need to alter or add any other privileges.
Choose OK
Now the backup agent should be able to use this SQL service account to backup the selected databases.
