I always defer to Tivoburkee for definitive answers on issues like this, but I don't think it's possible. I believe there may be a way to get the remote to work with external audio devices like AVRs, but not soundbars. The only way the remote would control the soundbar volume is if it was connected to the TV via eARC/ARC. I don't believe any TV can change the audio level coming out of its optical port.
One solution is to return the stick and get a Roku device that can be controlled via an IR remote (anything but a stick) and then use a universal remote that allows you to program the volume controls for the soundbar and the other controls for the Roku device.
