In private chats, you can only share your screen from the desktop application.
The online web browser version does not have the ability to share screens in chat.
You cannot share your screen in a Teams channel.
|Desktop version of Teams|
Web browser version of Teams
Start a video meeting by clicking the video camera icon.
Once in the meeting, choose the 'share screen' icon.
Choose which window or application you want to share with the video meeting.