Most standard stereo recording or live streaming software only pick off channels/tracks 1 & 2 from the USB B multitrack output.
Do you just want to send the what the main LR mix is to the live stream or did you want to create a separate mix for the live stream.
Using a separate mix would allow for some separate mix balance adjustments to the live stream feed and you can apply EQ and compression just for the live stream feed, you will want to use some moderate compression on the live stream feed upwards to a 5 to 1 ratio and don’t go overboard on the low end on the live stream audio.
Set the mix for post fade and to start with make it post all.
You can select the audio sources sent to the USB B tracks in the IO set up menu. Select track 1 and use the input knob to scroll through the selections, pick either L R or the mix if your building a separate live stream audio mix.