The button for screen sharing is located in the upper right corner, within the video. It can be found in the lower left corner of the video as shown on the image. Within the red circle is the button for screen sharing. Click on it.
Once you click on the button, a window will pop-up asking you whether you want to allow the use of your screen. Open the drop-down menu and locate the window that you want to share (you can share the entire screen too) and click on it and on "Allow".
That's it! You are successfully sharing your screen.