Mendix does not provide such an option out of the box.

You could try to implement it using some custom css and javascript like explained in this example. You could implement this as a custom widget or HTMLSnippet to the specific pop-up window(s) where you want the minimize functionality, but it's probably not very easy.

http://www.jqueryscript.net/lightbox/jQuery-Extension-To-Minimize-And-Maximize-Bootstrap-Modals.html.