In the "Showing new windows" there is a description to open a new window, but... if I like to close that window from inside the window. how do I go about that. I have tried a lot of stuff, and naturlly search the web but no luck yet!

how to dismiss it self?

NSWindow has a close() method that you can call from within an NSWindow subclass. If you want to close the window from an NSWindowController, you call the close() method on its window property. If you're in an NSViewController, then you can get to the window from the view, using view.window?.close(). Checkout the code in this GitHub repo (specifically in Test/PopupViewController.swift) for a working example.