http://dnbible7sqskwppgaqohboxz7u5kzffjzdx3lxzpjtt3s6tzeiytb5ad.onion/bible/communication-methods/jabberxmpp/using_jabber/index.html
It is sufficient if you choose for example “check your email account” as a question and a random string like “Af!J}m” as the secret answer. Before you click on the “Authenticate” button, send the other user that secret answer through a secure channel first.