Enabling 'Application-Consistent' backups on Proxmox VE must be done through the Proxmox VE Webinterface.

To enable 'Application-Consistent' backups, follow the steps from article below:
Qemu-guest-agent - Proxmox VE

For more information regarding how 'Application-Consistent' backups work in a Proxmox VE environment, check the article below:
VM Backup Consistency - Proxmox VE