Originally Posted by
username
TSA (Taipei Song Shan) is another one. NH operates TSA-HND twice a day. Initially, United.com does not recognize it.
Now you can put it in but it tells you no one flies there. So, it is not just "adding the item to the dropdown" - something else has to happen internally for it to actually work.
I've had a number of itineraries where the system - unprompted - suggested a TPE/TSA connection for an award. It clearly knows at least some of the flights through there.