At the bottom of the presenter screen, click Share screen to begin.
You will then have the choice of sharing Your Entire Screen, an Application Window or a Chrome tab. After making your selection and clicking Share, your audience will be able to see your screen.
When you're finished, click Stop sharing either on the BrightTALK presenter screen or the Chrome pop-up on the bottom of your shared screen.
NOTE: Apple have introduced additional security measures for MacOS Mojave (2018) and subsequent OS releases. You will need to ensure that screen sharing is allowed to be used by 'Google Chrome'.
You can update your security permissions by opening 'System Preferences' and selecting 'Security & Privacy'. Navigate to the 'Privacy' tab, and choose 'Screen Recording' to reveal the option to allow 'Google Chrome' to control your screen: