When you get to the screen sharing step of onboarding, the below image is the first page you will see.
Click the blue Share Screen button on the right side of the screen. You will then get the below popup screen.
On the popup screen, choose the screen you would like to share for your proctoring session. Once you select the image of your screen, the Share button will become active and you can click it. Once your screen is being shared, you will get the page below stating your screen share was successful. Click the Next button to continue onto the next step.