First I am assuming that it is raw. That was part of my question after I tried the suggested command:Why do you want to have raw at all? Just stay with a storage system that supports snapshots.
Code:
root@pvemacpro:/dev/pve# qemu-img snapshot -c snap1 vm-104-disk-6
WARNING: Image format was not specified for 'vm-104-disk-6' and probing guessed raw.
Automatically detecting the format is dangerous for raw images, write operations on block 0 will be restricted.
Specify the 'raw' format explicitly to remove the restrictions.
qemu-img: Could not create snapshot 'snap1': -95 (Operation not supported)
Second, it's the file format Proxmox decided to use for the boot disk. It's not my choice...?