What information do you think is missing in the CLI help text?
Your assumption is correct. The secret is required in base32 format as with YK. Here is the information directly from the code:
def register_yk_hmac(self, slot: int, secret: bytes) -> None:
"""
Register a Yubikey-compatible challenge-response slot.
@param slot: challenge-response slot
@param secret: the secret
"""
According the Grand Unified Theory of Documentation, you might want to checkout this well written tutorial?