In order to do this you need to use either our desktop client (for Windows or macOS), or use Zaurus in one of the following web browsers: Chrome, Microsoft Edge, or Firefox. If you use Chrome or Edge, you will need to install the desktop sharing extention. You will be prompted to install this the first time you try to share the screen without it.
Once in the video conversation you will see a number of buttons in the upper left corner.
Use the left one circled in red below to start sharing your screen.