Trigger backup from client from command line

I use urbackup to backup a Windows server and I use postfilebackup.bat to poweroff the server after the backup.
It works very good but the backup is scheduled and could not be predicted during the day.
I would like to trigger it a specific time, 08h00 for example.
I use incremental and full file backup using Urbackup parameters and if possible I would like to keep that.

How can I do ?