Click on a VM and jump to the tab "Backup", there you can create backups right away.
It should be possible to trigger the backup jobs manually.
Please add a button for this in the gui or at least via cli.
https://forum.proxmox.com/threads/mirroring-backups-to-multiple-storages.21061/#post-211578
I've had a talk with the Team, it's technical impossible to implement this atm.
But you have a Backup button on each node/vm.