To share your screen as a tester, or to have your tester share their screen to you as a researcher, you'll need to simply click the 'Share your screen' button to the bottom left of the video feed.
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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 share by clicking on it, even if you don't have multiple displays connected.
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While sharing your screen make sure you don't have sensitive information visible to others.