Create a link on the SSO Provider site that points to /session/sso. Set the return_path parameter to send the user to a particular page on the forum. If the user is already logged in on the SSO provider site, they will be automatically logged in and redirected to the return_path. If the user is not logged in on the SSO provider site, clicking the link will initiate the SSO login process.
<a href="https://forum.example.com/session/sso?return_path=https://forum.example.com/t/the-topic-slug/1234">Your Link Text</a>