The Fn key is typically handled specially by the EC (Embedded controller) in laptops, by itself it usually doesn't generate a keyboard scancode that is detectable by the OS.. but some combinations might.
Brightness control is often done transparently, but if your particular laptop generates a scancode that is usually interpreted by a driver/app running Microsoft Windows you might be able to find a way to simulate the effect using xgamma(1), or perhaps a 3rd party wrapper..
xbrightness.
You can run xev to see if the Fn+F8 key generates anything, if it doesn't, it may be handled by special ACPI events.. which perhaps may be unsupported for your laptop.