The following code worked for my LG TV (55’’ with webOS).
entity_id: media_player.sofita_tv
command: system.launcher/launch
payload:
id: "youtube.leanback.v4"
contentId: "nEi2Udjc0lQ"
where contentId is the youtube video url.
It does work!
The only disappointment was that TV asks you if you want to play the video as it was called outside youtube app.
Disappointment indeed as I was planning to create some NFC tags for my son (he is 3 years old) to play his favorite cartoon without parental control 
If anyone has found a way to bypass this window on TV it will be ACE!
Looked into youtube settings on our TV but there is no such an option.
Anyhelp would be much appreciated.