If you want to stay within ST, you can use GitHub - joejarvis64/j64.Harmony: Harmony Remote Control. His approach requires an internal server, but his instructions are pretty clear and he’s on the boards here. See link below.
There’s also an Alexa skill that someone developed called alexaHarmonyApp that connects Alexa directly to the Harmony hub, bypassing ST completely. It requires quite a bit of setup and code tweaking to personalize it. I was able to get the framework set up in one evening, but I spent about a week hacking away at the code to get all of my commands working correctly. I can give Harmony individual commands like “volume up 5” or play, pause, etc. It requires an internal node.js server running in your local network to receive a command from the Alexa skill and pass on to the Harmony hub, as well as a self-signed SSL certificate. You also need to sign up for a service like noip.com or similar to get a FQDN that points to your external IP address, and enable port forwarding in your router and open the same port in the firewall of your internal server. Kind of a PITA but once it’s set up it is very nice to be able to do individual commands by voice rather than just full routines.