Nothing to set up really. Receiving a call, I just tap the screen on the watch and hit the answer button. Do the call, press the hangup button. The call audio streams into my HAs (Resound Forte 8), but I think it likely works the same way with the KS8s. The watch has a mic (and a speaker, although the speaker is not used when the HAs are enabled on the iPhone).
The phone app and the Resound 3D app on the phone are both mirrored to the Apple Watch, so pretty much anything you can do WRT a phone call on the phone, you can do on the AW. It’s a lot easier to do than to explain.