I had a similar requirement. The Backup Exec agent is passive, and cannot invoke a backup job.
My solution is a little complicated- I created a backup job using BECMD, then created a batch file on the backup server to run the command.
On the client, I created a batch file that calls the batch file on the backup server (using psexec.exe from Winternals).
I then scheduled the client backup using the clients' Windows Task Scheduler- using a domain admin credentials as the runas.
The user can run a backup on demand by going to task scheduler and clicking run now. I can also have scheduled backups run without having to worry about the user turning the workstation off and causing a backup failure (If the workstation is powered down, the backup job does not run- as opposed trying to run and failing)
This is just one (my) method- perhaps there is an easier way....