how to connect Apple TV HD to a soundbar with optical audio ?

It should go through either the TV assuming it has an Optical output or if you use a receiver you can use that too.

If your TV does not have optical output or you don't have a receiver that support that, then you are out of luck and have to rely on the TV's speakers.

Unless the monitor supports Optical out, then you are out of luck and need to upgrade your devices like using a real TV.