Not supported.
Apple keyboard sends keycode for Fn unlike most of other keyboards.
-I think you can send Apple Fn key using Apple venter specific Page 0xff00 and usage 0x0003. But you have to change HID Report Descriptor for this, of course.
+I think you can send Apple Fn key using Apple venter specific Page 0xff01 and usage 0x0003. But you have to change HID Report Descriptor for this, of course.
https://opensource.apple.com/source/IOHIDFamily/IOHIDFamily-606.1.7/IOHIDFamily/AppleHIDUsageTables.h