How to turn off sound bar/hifi that's connected optically on LG TV

[cb4332]

Hi guys, I have an older received (Arcam AVR300) that only support optical input, so I was able to add it manually via Connected Devices, and can control the volume and mute, however I can't turn it off quickly (can only do it via the "..." button and selected sound bar and then power). This work, but is it possible to map this to lets say the top box on/off button or even some other button? Maybe even some cable I would but to make this work. The old remove is dead so I need a way out and the receiver is old and I don't want to keep it on always. Thanks