ERDisk.exe (C:\Program Files\Quest\Recovery Manager for Active Directory) is the command line tool used to start a backup session. You need to pass the Computer Collection ID to ERDisk.exe in the form or an argument, such as ERDisk.exe -ID:x.
To determine the Computer Collection ID, you can temporarily create a Windows Scheduled task using the Schedule option of the Computer Collection Properties. Edit the task that gets created and view the Argument under the Action section.
You can also obtain the ID from the ERDiskAD.mdb file. You will need to first download and install SQLiteBrowser:
http://sqlitebrowser.org/. Open the mdb file using SQLiteBrowser and select the Browse Data tab. Click the dropdown and select Folders. The FolderID is the ID number you will need for the corresponding collection name for that record.