I am quadriparetic would Limited hand control, so I do use hands-free control of the Harmony for all buttons, including pause, rewind, volume up, volume down.
The short answer is that unless you want to set up your own server and run it, you do have to create a Harmony activity that will do that function. Once you have the activity working on the Harmony side then it’s easy to trigger it from the SmartThings side or from IFTTT.
there are some quirky things about it, and I don’t know that most able-bodied people would want to bother. It’s pretty clunky.
Because harmony will not let you run the same activity twice in a row, for example, I actually have two volume up activities, and two volume down activities. That way I can swap between them.
For something like pause, I have one activity that does the pause. And then I have to immediately activate the regular “watch TV” or “watch DVD” activity so I get back control of the other keys.
So it’s not pretty, but it can be done. You’ll find discussion in the harmony/Alexa project report near the end, including sample screenshots for the harmony activities. Even if you’re not using Alexa, the same activity set up would apply.
There’s also somebody who’s got a project report on running their own server and setting up a CLI to harmony which is more flexible, but requires a lot of technical sophistication to set up and run. So I don’t know if that something you be interested in: