Google Chrome version 72 supports screen sharing (you do not need any additional browser extension).
Google Chrome versions prior to 72 does not support native screen sharing. Hence Cisco provides a Chrome extension to enable that functionality.
During a meeting when you click on the "share" icon, you will be prompted to enable this extension. Follow the instructions on screen to enable the extension.
Alternatively, you can enable this by clicking on the menu option: More tools > Extensions.
Refer to the this Cisco Meeting App WebRTC important information document for more information on supported browsers.