|YouTube Cast Button|
Finally I noticed that my smart TV was off and may be it needs to be "on" if that cast button to appear on youtube videos. Voila ! the trick worked.
Additionally, make sure your smart Tv is connected to wifi and you have sync your TV for screen mirroring. Let me know if it worked for you.
Video screenshot courtesy : Google Channel - https://www.youtube.com/watch?v=O-XidwKsKAE