In order to share your screen during a Teams meeting, click the Share button on the top menu.
Next, select the screen sharing option from the list. Options are: Share your entire screen (in red), share a specific window (in green), or a whiteboard. Other options allow to share a powerpoint or excel file live, a file from your OneDrive account, or a file from your computer.
Clicking "Window" will display a list of windows available to share (example below)
After you are finished sharing, click "Stop Share" at the top center of the screen.