When you enable streaming events to other destinations, all new events that you create will automatically be set to go live on both Livestream and your other destination(s) by default.
However, events created prior to enabling simulcasting in your account setting will need to be turned on manually. You can control your simulcasting settings on an event-by-event basis.
Navigate to the event for which you wish to turn simulcasting either on or off. Click the Editor icon to open the edit drawer.
You will see a toggle switch next to your streaming destination (Facebook, in this example). If it's green, it means simulcasting is enabled for this event. Click it to turn it off.
When it's gray, simulcasting is off. Click it again to turn it on.
This can be done whether or not your event is live.