From what type of device?

If using an iPhone/iPad, then simply use an Airplay v2 connection to the Port, or maybe add a Bluetooth Rx to the Ports RCA line-in and send the YT audio via that link. It may not play in lip-sync though with any video on screen, via the line-in option, but perhaps that may not matter, depending on what you’re attempting to achieve here?

The Sonos Port has an analog line in. If your TV has an analog line out, you could connect the two, although it’s really not designed for it. There would be, of course, a 75ms delay minimum on the analog line in, so lip sync would be a challenge.

Honestly, you’d be a lot better off with a Sonos device actually designed to connect to a TV set via a digital line in, such as the Sonos Ray, the Sonos Amp, the Sonos Beam, or the Sonos Arc, all of which would be easier to connect, using that digital line in, and do away with that delay you’re going to run in on the analog line in.

I now see you mention TV in your opening post - If trying to send the YT audio from your TV, you will likely be better off adding a Sonos HT product to the TV and (if still required) grouping that chosen HT product with the Port instead. 

Another viable option, is to use an Apple-TV connected to the TV and send its audio-out to the Port over Airplay v2.

Thanks Ken. The apple TV audio out to the port via airplay worked.