Can you elaborate on what exactly does not work when you use your Playbar like this? No sound at all? No sound from speakers other than the Playbar? No sound from YouTube only? Where is YouTube playing from (app on your TV, Chromecast)? 

You need to set the input to the Playbar as “TV”. There is a setting too, TV Autoplay which will switch to tv when a signal is detected 

The TV may also need a setting changed, so that it disables the built-in tv speakers and sends the audio signal to the optical output. 

Surely to get any sound from your tv to your Playbar it needs to be connected as you describe? 
But you don't need your TV to play YouTube music to the Playbar. Add YouTube Music as a music service on the Sonos app and play it from there, selecting the Playbar as the speaker.