Calling a custom GUI for built-in plugins

What is the proper method for implementing a GUI for built-in plugins and having them open with

Sorry, I missed this.
showWindowExplicitly simply calls though to the PluginWindowState::showWindowExplicitly().

To actually show the window you need to override UIBehaviour::createPluginWindow. There you can examine the PluginWindowState arg and dynamically cast it to either a Plugin::WindowState or RackType::WindowState to get the Plugin or RackType associated with it and then create the UI Component and window and return it.