/etc/pve/qemu-server/<vm-id>.conf
To configure file from the CLI:
Ensure you are logged in as the root user.
nano /etc/pve/qemu-server/<vm-id>.conf
args: -drive 'if=pflash,unit=0,format=raw,readonly=on,file=<pathto>/OVMF_CODE.sw.fd' -drive 'if=pflash,unit=1,format=raw,file=<pathto>OVMF_VARS.sw.fd'