how to add sound in window capture screen

i d like to do a live on Facebook and switch from my webcam to youtube by exemple … with the sound of youtube in my tv program (Facebook live)

You will need to run a 3rd party application to capture the sound of that particular app (e.g. Soundflower or Loopback ). Such an app will provide the output of an app as an audio source to other apps like mimoLive.