I found solution for my keyboard... When it pops out to put PIN just tipe 4 digit number and click connect and then press the same 4 digit PIN code on your bluetooth keyboard and press enter/return
Hope you found it useful. Please let me know.
It did the trick for me, pairing an Apple Wireless keyb to windows 10. It was driving me crazy. Strangely, a day before Windows told me to enter "XXXX" PIN on the keyboard and press Enter. Since today no such option.