With the recent MacOS Catalina Software Update (Summer 2020), Forum requires permission to use the Share Your Screen function.
Forum on Chrome
You will see a popup appear asking for your permission to access your computer screen on Chrome in the System Preferences. Click Open System Preferences to go into your computer settings, to confirm this.
Click Open System Preferences to go automatically to your System Preferences to allow this. If you click Deny, you will have to go to your System Preferences manually and do this.
Within the Security & Privacy, within the Screen Recording section, select the Privacy Tab, to see Chrome on the list of apps that need permission to record the contents of your screen.
Click the checkbox next to Chrome to approve this. Chrome will have to quit to allow this, so click Quit Now.
Open Chrome, go to the Forum Classroom, and you will now be able to share your screen.