Storyboard: Making Reference to Window Controller Property

I am making the transition from iOS development into OS X, but am having a bit of trouble understanding some of the relationships between NSWindowController, NSViewController, and how they work with Interface Builder.

I have a NSSplitViewController subclass that has a variable for sidebar NSViewController. I don't see a way to make that connection as an IBOutlet in Interface Builder? Am I missing something?

This is what I am doing thus far. If there's a better way, I'd love to learn.

class PrimaryWindowController: NSWindowController
{
    var splitViewController:PrimarySplitViewController! {
        didSet {
            splitViewController.persistenceController = persistenceController
        }
    }

     override func windowDidLoad()
    {
        super.windowDidLoad()
      
        if let split = contentViewController as? PrimarySplitViewController
        {
            splitViewController = split
        }
}

}

Thanks for the assist.