SPDIF to Toslink decoder. You want a decoder vs a converter because most SPDIF optical outputs like from the TV are only going to be PCM audio and a lot of the coverters have issues with that, where a decoder can keep the 5.1 sound and pass it to the soundbar to process. A good one will probably run $50.
Have to decide if it's worth it or take it back if you can and get a soundbar that has hdmi inputs.