I just tested with v4.2.0 and the skey value I passed in at the command prompt is shown in the registry. You did surround it with quotes? Also, are you looking at HKLM\Software\Duo Security\DuoCredProv or HKLM\Software\Policies\Duo Security\DuoCredProv? The first location is where the installer puts the values it received, and the second location is where the GPO puts its values. If you have a Duo GPO that doesn’t configure the SKEY, then that POLICIES key wouldn’t have a value for the SKEY.