To set your input device as StageSound for a Facebook livestream ensure StageSound is opened and is turned on. Then return to Facebook and start a livestream.

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Scroll down to the Stream setup options, and in the dropdown menu next to the microphone icon select StageSound. 

 

When you have finished livestreaming and no longer want to use StageSound on Facebook then follow the above steps again to access this menu, this time selecting your usual microphone.