Declare custom class (from gha) in script?

I've tried searching for this and hope I'm not rehashing something already covered:

I have created some C# classes in Visual Studio and used them to make some GH components (GHA) which all seem to work fine. I would like to be able to declare and instantiate those classes as new objects (so not passing them into or out of the script as a parameter) in the script as a tool to do stuff, then output normal GH geometry as a parameter from the script. Is this possible? My assumption was that I need to turn on a reference to that GHA, but I can't figure out how to do that. And I can't find it in the documentation or threads I've seen here.

Many thanks!

Jason