If your laptop has double uses assigned to its function keys, then there should be a key in the bottom left that says "Fn" or something similar. If you hold that button down while you press F8, then that should work. (Assuming that your focus is on the Pokémon Reborn window, of course)
If that doesn't work, then there should be a "PrtSc" (Print Screen) key right above your Backspace key, which takes a screenshot of your, well, screen. If you go into MS Paint or something, you can paste this image there and then crop it and save it as an image file. (This latter method is how I create my Reborn screencaps)