I've also looked for a solution to this. In Zoom I can share a window and I put my OBS Program in a windowed viewer. I don't know if that is available in webex. For other video conf programs, I hang a USB3 encoder on the mac's HDMI port and I put my OBS program out full screen to it. I also plug the encoder into a USB port on the mac and it becomes another webcam source (that happens to now contain my OBS program.) I use that source as the webcam for whatever videoconferencing software I am using and the other attendees see my OBS program out as my webcam. My preferred method is to run OBS on one mac and video conf from the other but use the same basic approach with encoding the OBS output.