Manual:CHR ProxMox installation
Create new guest with the system disk and other devices as required.
Then you have to manually upload the CHR disk on the ProxMox host.
Use SCP or comparable tool as that will use SSH for the upload and does not require any additional configuration.
Either scp the file up and then manually edit the VM's .conf file or replace create system image file used for booting the guest.
Local storage on ProxMox is in /var/lib/vz. There should be an images subdirectory with a directory there for each VM (named by the VM number). You can scp the files directly there.
For adding the existing file to the VM, edit the VM's .conf file directly. Look in /etc/pve/qemu-server/ for a file with the VM number followed by .conf.