The easiest way is to simply ask here. We understand the terminology and know where to find the codes. OFF is Nec1, device 4, function 197. I have no idea what format your HTC app takes, so I can't help with that. I can tell you that if you simply pick a different LG model in the Harmony database, it will likely have discrete off. Also, if you have a cable TV remote, most can send any discrete command if you use the appropriate code (00176 in this case).

The fact that 128.126 (function 8) also works is probably just a fluke. I have never seen any LG TV use that kind of signal.

No LG remote is going to have discrete on or off.

Lastly, you can simply ask logitech support to add discrete off to your profile.