Meeting with Single Application Window without Blank Desktop

I have a high resolution screen (4K) and only want to share a particular application window to show someone something. I can select the window to show ok but the person at the other end sees my whole blank desktop as well which doesn't really work well as it doesn't fit on their screen. The end result is something that isn't readable.

Isn't there a way of telling TV to make the viewed window the whole screen and exclude the blank desktop from the viewing? I can't see the point of transmitting a blank desktop.

I did try the advanced option and not select the desktop but it doesn't make any difference.

Thanks