I’m aware that it’s not advised to pass data from the front channel back to Auth0, to the /continue endpoint, via URL (query string) parameters. However, this is what I need, to send something non-sensitive. I don’t want to go to the hassle of creating and signing a JWT to send back, as this article describes.
I have reviewed all the available properties in the post-login event object and found the event.request.query property which returns query parameters passed in a request.
I have tested this and confirmed you can access your query parameter by calling event.request.query.foo.