Refer to this article on how to add a custom RTMP destination.
If your RTMP destination isn't working, check to see if you have provided the necessary permissions and that your Stream Key and Ingest URL for the Destination you're wishing to broadcast to are correct. If the destination does not work, it is possible that your Stream Key may need to be reset and then updated in Lightstream Studio.
If you still have issues broadcasting to a destination, please let our support team know so that we can check into that for you however, it should be noted that not every custom RTMP destination is supported by Lightstream and we cannot guarantee being able to broadcast to streaming platforms that we do not directly support.