To restore the use of the F8 key, connect your USB keyboard directly to the computer's USB port.
Apple Thunderbolt Display (27-inch): F8 key does not work using Windows with USB keyboard connected to display
You may notice that when using Windows and a USB keyboard that is connected to a Mac via the USB port on an Apple Thunderbolt Display (27-inch), the F8 key is not recognized by Windows when you attempt to accept the Terms and Conditions agreement or start up into Windows Safe Mode.
Information about products not manufactured by Apple, or independent websites not controlled or tested by Apple, is provided without recommendation or endorsement. Apple assumes no responsibility with regard to the selection, performance, or use of third-party websites or products. Apple makes no representations regarding third-party website accuracy or reliability. Risks are inherent in the use of the Internet. Contact the vendor for additional information.