I couldn't make this work. Accessing the nbd freezes the shell. Here is what i did:
export PBS_REPOSITORY='root@pam@192.168.50.2:stor1'
export PBS_PASSWORD='my.password'
export PBS_FINGERPRINT='AA: (...) :18'
root@prx002:~# proxmox-backup-client list
root@prx002:~# modprobe nbd
root@prx002:~#...