The only way I found to capture these buttons is if you have a Hue Hub connected to the Harmony. If you have, then depending on your remote you can either allocate a Hue scene (Elite remote) or a single bulb (Companion remote) to the button you showed and then by modifying the driver code with your specific event codes you can trap those events and get it to trigger a button press in HE. It's a PITA and you have to effectively waste a Hue bulb just as a trigger, but it does work. At least if you have an Elite you can set up 4 scenes that just use the same hue bulb and just call them different names to get access to all 4 buttons. With the Companion you need a different bulb for each button you want to use. I haven't trapped the +/- button presses in either case, because with scenes they don't do anything and for bulbs they only work on the last bulb switched on. Logitech's set up of these buttons is bizarre and even If you don't use the +/- buttons you can't allocate them to A/V commands as that overrides all of the home control buttons. If you look in the other thread linked above, there are some details and I am happy to PM you my modified driver code that traps the button presses if you want to have a go.
Thanks, I just used this since I don't have a Hue. GitHub - ruricu/SmartThings Setup a Scene in HE, published Scene to Maker API, now I can call that local Maker API url through ST Button that I can then assign to Harmony Home Control buttons. I could also probably use the Harmony Hub direct integration and RM to create a rule to "dim lights" if it's in the evening and a certain activity is started, but this is the quickest way to get the same buttons to do same things. If someone has a better option, please let me know. Unfortunately that button device runs in the cloud, but it can still call the local HE API endpoint.
Any change you would tweak your momentary switch driver to allow us to insert json parameters?
There's always a chance... 
But somehow, I am guessing this would snowball into a bunch of requests to handle the response, etc...
Rule Machine allows http calls these days. Wouldn't this be an easier, built-in approach?
Yes but RM only has GET and POST... I need PUT as I am working with the HUE API.
webCoRE can't do this?
I use it for my hue for very specific colors.
