Thanks. Worked with my M Edge BT KB on Win 10, 1903 witn an ASUS 4.0 BT USB dongle. I am just paraphrasing your reply for clarity:
When the master device (in my case Win 10) pops out asking you to put in the PIN without initially proving one, just type any 4 digit number and click connect, and then press the same 4 digit PIN code on the slave (Bluetooth keyboard in my case) and press enter/return