This means that townnews would be the SSO provider for your Discourse site. To use Discourse’s implementation of SSO you need to be able to add code to the service that is functioning as the SSO provider. That code needs to hook into the service’s login process. You can find details about the code that needs to be added here: Official Single-Sign-On for Discourse (sso).
For a working code example, have a look at how our WordPress plugin implements SSO: https://github.com/discourse/wp-discourse/tree/master/lib/sso-provider.
It may also be possible to log users into Discourse via Townnews with OAuth2. This would be possible if Townnews can function as an OAuth2 provider. There are details about setting up OAuth2 logins with Discourse here: OAuth2 Basic Support. Before spending too much time on that, it would be good to confirm if Townnews can function as an OAuth2 provider. It should be possible to find that out from their documentation.