BluetoothAuthenticateDeviceEx

[DllImport("Irprops.dll", SetLastError = true)]
public static extern uint BluetoothAuthenticateDeviceEx(
   IntPtr hwndParentIn,
   IntPtr hRadioIn,
   ref BLUETOOTH_DEVICE_INFO pbtdiInout,
   IntPtr pszPasskey,
   uint ulPasskeyRequirement
);

Microsoft documentation: Link