Using a 3.5mm connection I would plug into the ALT OUT jacks with a adapter cable that goes from two 1/4 inch plugs to the single 3.5mm plug, ALT OUT has a level control on the front surface.
In the IO patching you can select the source to the ALT OUT.
Lot of options for a live stream audio, it’s been discussed here many times. Using the main LR mix, setting up a post fade aux or using a matrix mix out and a couple other ways as well.
What model mixer do you have?
You could possibly set up your system and use the USB B output connection to feed directly into a computer.