To share your screen as a tester, or to have your tester share their screen with you as a researcher, you'll need to click the 'Share' button to the bottom left of the video feed.
Upon clicking the share button you'll be presented with the browser's pop-up window. You need to select the actual screen or windows you want to share by clicking on it, even if you don't have multiple displays connected.
While sharing your screen make sure you don't have sensitive information visible to others.