Just connected my Apple Pro KB and was unable to find a key to do it. F13, F14, F15 all didn't work. F13 is in the same physical location as the print screen key on my Logitec wireless kb that I use for Windows. So my guess is on Apple branded keyboards there isn't one, you might be able to get a utility that maps that key or maybe you can set it in Windows yourself I'm not sure. However, outside of full screen apps you are probably just as well off using the Snipping Tool.