To set your input device as StageSound on YouTube ensure StageSound is opened and is turned on. Then return to YouTube and start a livestream.
The Webcam stream info should automatically open, click on more options.
An option to select camera and microphone will appear at the bottow of the window. Use the dropdown menu next to the microphone icon to select StageSound.
When you have finished livestreaming and no longer want to use StageSound on YouTube then follow the above steps again to access this menu, this time selecting your usual microphone.