Add a NSVisualEffectView below NSTextView

I want to add a NSVisualEffectView inside my NSTextView, however when I add it, the text is below the NSVisualEffectView, so, how can i add the NSVisualEffectView below the text?

My code for OS X:

class myTextView: NSTextView {

    override func awakeFromNib() {
        let visualEffectView = NSVisualEffectView(frame: NSMakeRect(20, 20, 30, 18))
        visualEffectView.material = NSVisualEffectMaterial.Dark
        visualEffectView.blendingMode = NSVisualEffectBlendingMode.BehindWindow
        self.addSubview(visualEffectView)
    }
}

Maybe this will help you:

class MyTextView: NSTextView {

    lazy var visualEffectView: NSVisualEffectView = {
        let visualEffectView = NSVisualEffectView()
        visualEffectView.material = NSVisualEffectMaterial.Dark
        visualEffectView.blendingMode = NSVisualEffectBlendingMode.BehindWindow

        return visualEffectView
    }()

    override func awakeFromNib() {
        super.awakeFromNib()

        self.drawsBackground = false
    }

    override func viewDidMoveToSuperview() {
        super.viewDidMoveToSuperview()

        if let containerView = self.superview?.superview?.superview {
            containerView.addSubview(self.visualEffectView, positioned: .Below, relativeTo: self)
        }
    }

    override func resizeSubviewsWithOldSize(oldSize: NSSize) {
        super.resizeSubviewsWithOldSize(oldSize)

        if let superview = self.superview?.superview {
            self.visualEffectView.frame = superview.frame
        }
    }

}

It's a text view in a scroll view and clip view so you have to get its superview.