I tried using mkp224o to generate a Tor vanity address for my Bitcoin Core node. It output something like this:
./mkp224o -y satosh
hostname: satoshf47rudte4hkkhhpz7sndbizctnvh3qcmdsaegjaprobb5mt6id.onion
hs_ed25519_public_key: PT0gZWQyNTUxOXYxLXB1YmxpYzogdHlwZTAgPT0AAACQJukcvPxoOZOHUo535/JowoyKban3ATByAQyQPi4Ieg==
hs_ed25519_secret_key: PT0gZWQyNTUxOXYxLXNlY3JldDogdHlwZTAgPT0AAADwgEV7/yRWZksAC7dZCHRWx0GQVn7q3Wki2rEmQd1DaBtwbyaym1Ohy+572xy49hnGgJyh2eWdEaaOs/ko0nkw
I assumed I could just edit onion_v3_private_key
and put the secret key in there after "ED25519-V3:", but it didn't work, and the generated key also looks longer than the one originally in the file. The error in debug.log was:
tor: Add onion failed; error code 512