Hello Robert,
In order to use a VST plugin you need another program, such as Reaper. Currently Hauptwerk does not directly support VST plugins.
You can find a tutorial here: https://www.hauptwerk.com/news/tutorials-applying-software-reverb/
When I started to use reverb plugins, I was very disappointed with the result until I discovered that it makes a whole difference when reverberation is routed through a separate channel. I created a short description with a link to a very helpful YouTube video here: https://www.hauptwerk-organ.com/reverb/.
Hope that helps!
Best,
Chris