If you don’t have a Yubikey, you don’t need a hex string.
As @nku explained, a twenty digit random value converted to a SHA-1 hash with base32 is sufficient.
E.g. like this (source: comment from Anonymous on 22. August 2023 - 19:16):
Generate a 20-byte random string with:
dd if=/dev/urandom of=/tmp/nk bs=20 count=1
Encode this and add to your nitrokey in a slot (slot 2 here):
nitropy nk3 secrets add-challenge-response 2 $(base32 /tmp/nk)