The library's one setting, read from its own otp_app:

config :managoat_runner, host: MyApp.RunnerHost

host is the module implementing Managoat.Runner.Host. There is no default: a consumer that forgets it gets an error naming the key, not a connection that registers nowhere and a call/3 that finds no runner online. The WebSock init map may carry host: too, and that overrides the configured one for that connection (it is what lets a test run two hosts side by side); nil counts as unset either way.