I’m not sure why you haven’t got a response here. Unless others are waiting for you to do some simple searches on this site. Anyway, I hope this helps as I’m no expert on SSO.
SSO or SAML connection will be by setting an SSO/target URL at Discourse.
Have you read these topics on Discourse and off-site SSO?
Did you find the SAML plug-in?
https://meta.discourse.org/t/adding-saml-sso-as-an-option-but-not-enforced/47035?u=remah
And here’s a comment about preferring SAML over CAS. Interestingly enough, by the author of this software to help with CAS to Discourse:
https://meta.discourse.org/t/cas-auth-via-the-sso-api/23381/9?u=remah
Much like the comment above. Note that has useful explanations unrelated to Ruby on Rails: