You could create an extension inside Flarum that reads the token you provided, relate that to a user and log them in. If you host your forum on the same domain as the react site, you might also read a cookie whenever a user hits the forum by adding a Http middleware to read the cookie, verify the user and log them in.
Please stop spamming around and try to resolve this issue. If you have no idea where to start, it might be best to research the topic of SSO. It seems complicated, but it really isn't.
There's also a passport extension that works perfectly with any Oauth2 authentication server. If that helps, it wouldn't be SSO, but it would offer a unified authentication method.
