This links to a github repository for an add-in that does exactly what you are asking about. The add-in installs a ribbon panel where the"KeyNote Editor" button on the panel starts an external tool.
https://github.com/akseidel/RevitAddinKeyNoteSystem
The AppKNS.cs file is where you find the code that builds the ribbon.
The OpenKeyNoteSystemCommand.cs is where you find the code that starts the external application.
aks