I got it working! Thanks very much @simon.
Now when someone comes into the forum; clicks the blue Login button; logs into wordpress by typing in their username/pw; they are redirected back to thread they were at in Discourse.
I think this is going to be a huge win for people who come in via the Discourse emails e.g. and want to participate in the thread. Previously, they’d lose track of where they were since they didn’t get redirected back to the thread.