create the page that lives at url B (https://example.com/pageb in this example). It won't be hosted by FusionAuth.
add url B to the list of Authorized Redirect URLs in your OAuth configuration.
That should work. There are some other alternatives, however. If you generate the "Forgot password" page without a client_id, you won't end up logged in after a password reset. Instead you'll end up at the "OAuth password complete" page, which you can design in the theme editor.