That’s not how it works. If your pc/laptop has an hdmi out port, hook it up to your tv. It has nothing to do with the cable box. If you have your cable box already attached to the tv, why do you need to stream to it?
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If you have one of the xfinity X1 boxes, it should have apps for most of the internet streaming services (Amazon, Netflix, Youtube, etc.) to use without the computer connection. If you want to watch xfinity content on your computer, go here: https://www.xfinity.com/stream/