You must be set as "Presenter" to share your screen. You can see if you are presenter in the User list if there is an icon of a presentation board on the left of your name.
Click the Share your screen button at the bottom centre of the virtual classroom.
In the next popup window, select which part of your screen to share; Your Entire Screen, a specific Application Window, or a specific Browser Tab (Chrome, Safari, etc).
Make sure to click on the actual window to share so that it is highlight in blue.
Click "Share."
See attached screenshots to help point you in the right direction.
