You’ll need to install the Community app called Hubitat Package Manager (HPM) if you haven’t already done so.
Here is a link to the community page that explains how to install it:
Once you have HPM installed, and from within HPM, you can install the Shelly Uni driver using the "From a URL" option in HPM, here: https://raw.githubusercontent.com/tomwpublic/shelly_experimental/main/packageManifest.json
