Bluetooth pincode for pairing

I have an Anker A7721 Bluetooth keyboard. I did try pairing on a laptop, but with the first try I failed in filling in the pairing code. When I tried again I got a blank field with the the instruction to give the pin code. But I have no code remembered or noted. I tried 0000 or 1234 but the pairing doesn't happend.
Pairing with android devices is no problem. What do I do?

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