I think I there were two things wrong.
1) Out of frustration, I went ahead and installed the github pull request builder plugin. I was then seeing messages in the jenkins log that
it was checking the PR but nothing got built.
2)After uninstalling the github pull request builder plugin, I figured out that the endpoint for the branch-source-plugin PR builder hook was incorrect. The
correct endpoint was http..../github-webhook.
Bottom line, you don't need the github pull request builder plugin if you are using github branch source plugin, it just needs to be properly configured.
Thanks Jan for the response.
Tom