The driver situation with Apple Bluetooth wireless keyboards and Windows 10 is horrible, even with the latest BootCamp drivers. Fortunately, a workaround is available, if you're patient.
- Turn on the keyboard.
- Press and hold
Command + wuntil the keyboard light begins blinking, indicating the keyboard is ready to pair. - Use Windows Bluetooth settings to pair the keyboard, entering the same code (e.g.
123456 Enter) on both internal and external keyboards.
Pairing the keyboard is very trial and error. 9/10 times, Windows will complain that the keyboard is not available for pairing. Just keep trying.