You don't need to do this - the concept of "passing the ball" in Webex has been obsolete for years. All the person on the video unit has to do is start sharing, it will work. I just tested this on my DX80, while dialed into a webex URI, from the web GUI I could not assign the video system as a presenter, but on the DX I just had to choose to view my PC and then share it.
Try it - it will work.