What is the hex ID for Fn key
I got this from Key Codes (freeware)

Fn key down

Modifier Change
    Keys:       
    Key Code:   65535 / 0xffff
    Modifiers:  8388864 / 0x800100 ⓘ


NX_SECONDARYFNMASK / kCGEventFlagMaskSecondaryFn / NSFunctionKeyMask
        = 8388608 / 0x800000

NX_NONCOALSESCEDMASK / kCGEventFlagMaskNonCoalesced
        = 256 / 0x100  

Fn key up  

Modifier Change
    Keys:       
    Key Code:   65535 / 0xffff
    Modifiers:  256 / 0x100 ⓘ


NX_NONCOALSESCEDMASK / kCGEventFlagMaskNonCoalesced
    = 256 / 0x100

tbh, it doesn't really mean a great deal to me, I'm no coder... ;)