I need to re-configure redirect url after successful subscription. however, in the new portal V3, I cannot locate the place to do the configuration any more.
It is possible to configure the redirect URL in portal V3. The redirect URL is plan specific and you can pass the Redirect URL in the Edit plan option under each plan.
You can do this by navigating to Plans -> Choose the plan -> Edit plan -> Pass the URL in the redirect URL box. You can add the parameters under the Return Variables -> Choose the subscription ID.