Instead of recording a video, you might want to record a screen share as a post or reply. To do this go to Record and Share your screen.
You can then choose whether you’d like to share a Tab, Window or your Entire screen.
Once you have chosen what you would like to share, you will see yourself in the left corner and an icon with arrows. By clicking on this icon, you can reposition yourself to any position in the screen or remove yourself altogether. You can also choose whether you'd like to spotlight your screen or yourself.
Once you are ready, you can begin recording your screen share by clicking on the red circle in the middle.