It seems like you might already know how to fix that feedback loop or at least intrinsically what is happening, and though I can't speak so much to specific, ideal PC audio or WebEx configurations, I thought I might offer my take. It's such a common problem that perhaps attributing it to the vendor(s), etc., might make the problem appear less general than it is or that a better solution was not possible.
The feedback occurs when the microphone/input "picks up"(hears) the speaker/output that is amplifying/receiving the microphone in close proximity. In your use case, however, there's no reason for your computer's microphone to be routed to speaker at the same time: it would be distracting to hear yourself through the speaker likely and it creates feedback issues obviously. The solution is to change your audio settings somehow, hopefully within WebEx, if not possible there then at the System level, and if there's no system utility that will help you, look for a 3rd party tool (I've used Virtual Audio Cable on PC with success but found it lacking/difficult/overpriced compared to Soundflower which is awesome/easy/free which is what I would use on the Mac when confronted with the same issue). With either of these utilities the concept is the same, being able to create virtual audio routes between programs and devices so it's not such an all or nothing proposition where audio signals are implicitly going to the same place. I'd be surprised though if this major conferencing software doesn't have this implemented though, so I should probably start rambling, the world has bigger problems.
Anyhow, there has to be a way to not hear yourself, but sometimes it is more of a pain on the PC, at least if VAC is the best thing. If there's a will there's a way though so if anyone is so challenged and wanted a cheap guide, I just burned some minutes typing this. Adios amigos!