pleroma/docs/configuration/howto_mongooseim.md

Configuring MongooseIM (XMPP Server) to use Pleroma for authentication

If you want to give your Pleroma users an XMPP (chat) account, you can configure MongooseIM to use your Pleroma server for user authentication, automatically giving every local user an XMPP account.

In general, you just have to follow the configuration described at https://mongooseim.readthedocs.io/en/latest/authentication-backends/HTTP-authentication-module/ and do these changes to your mongooseim.cfg.

1. Set the auth_method to {auth_method, http}.
2. Add the http auth pool like this: {http, global, auth, [{workers, 50}], [{server, "https://yourpleromainstance.com"}]}

Restart your MongooseIM server, your users should now be able to connect with their Pleroma credentials.