To enable screen sharing during a video call please click the share button in the bottom toolbar.
You will then be shown a new window where you can choose what screen or window you would like to share. Please select an option and click on the share button. The below example is using Google Chrome although it is very similar on other browsers.
Problems using macOS:
In macOS 10.14+ (Mojave and newer) Apple introduced new Security and Privacy features that by default block access to certain features including screen recording.
To allow access to screen recording please follow these steps:
Click Apple Menu on top left of screen.
Click System Preferences.
Click Security & Privacy.
Click Privacy Tab.
Select Screen Recording from left hand side menu.
Make sure your web browser is ticked.