Share a part of the screen only

Hi guys, 

I would like to share a tiny part of my screen to the audience. 
I noticed that an option called "position on screen" exists, which seems to work, but it appears to be unable to be locked on one window only. It follows me wherever I go, which I don't want. 

=> I want to be able to share a position on screen, only for one window. Is it possible ?
Thanks !