http://ciisqbg45nggykdl6rjdrq3wc64csga4vkphu66qsi65mypeitqedoad.onion/pfsense
Then add another mapping after that for the OPT1 Interface, with source set to fd00:20:20::/64 (or whatever IPv6 network you chose for the LAN interface earlier), and Translation set to OPT1 Address.