Or use polling schedules on the client. However, define 'start the backup job'. If you want the server to initiate the job so the firewall sees it as an established session, I don't know if polling scheduling is enough to accomplish that. Test.
Scheduling a command means the client will initiate the backup. TSM will cause an OS process to start, which might be a TSM backup, but it'll be client-initiated because it will not be a scheduled backup in the strict sense of the word.