On a desktop browser, go to instagram.com, start creating a Live, and choose the “use streaming software / external source” option. Instagram reveals a Server URL (rtmps://…) and a Stream key.
In your Restream Portal (/stream) → Add destination: paste the Server URL into RTMP/RTMPS URL and the key into Stream key → Add.
Start your encoder pointing at your Expanse ingest (rtmps://live.expanse.io…).
Back in Instagram’s Live Producer, click Go live.
When finished, end the Live on Instagram and delete that destination in the portal (the key is now expired).