There is an official integration. In order to use that one, you must first create a Harmony “activity” using Harmony for everything that you want to do.
These activities will then be exposed to smartthings as virtual switches, so that you can use smartthings automations to start the harmony activities by turning on the switch created by the integration.
This is the official method.
https://support.smartthings.com/hc/en-us/articles/205653834-Logitech-Harmony-Home-Hub
Unofficially, some community members have created a method where you use an additional device as a “man in the middle“ server, typically a raspberry pi, and with that method you will be able to access individual harmony buttons, not just activities.
However, it requires significant technical skill to set up, as well as the additional device. Also, it will only work with the V2 classic version of the smartthings app, not the newer version. So it is not as popular as the official integration, but it does exist if you are interested in that approach.