Within the Jetway Configuration Tool I would like to disable the Jetway of the VHHX Kai Tak Airport but It`s not working for me because the icao "VHHX" is not not listed (See Screenshot!). How can I fix this?
There's nothing to fix and nothing you need to do. What the GSX Config panel does, is to prevent Jetways on a 3rd party airport to be overwritten by a GSX jetway replacement file
made for the default airport.
Since VHHX doesn't exists in the default scenery or, more precisely, didn't existed before SU11, there's no GSX jetway replacement file for it, so GSX won't replace jetways there in any case, meaning there's nothing you need to do to prevent anything, since the replacement is not going to happen in any case.