Chris the Pnt Scn normally captures the entire screen.

To capture only the active window you can use an Alt key and the Pnt Scn key. Is the Alt key possibly what you mean rather than the fn key.

I have an HP C610 laptop (yes it is an old one it works the same as a desktop computer. Pnt Scn to capture the entire screen and Alt+Pnt Scn to capture the active window.