I’ve done this by making a popup that uses a custom titlebar instead of the built in titlebar.
First set Titlebar Display Policy to Never.
Make your own titlebar for the window. It can be whatever you want, a label, a rectangle or other shape.
Next add this code and the custom properties as posted by user jaepark. Add the mouse pressed and dragged events to the titlebar only, not the root container.
This will make your window dragable to anywhere in the client (including off screen but you can handle that if needed.) And you also get to make a titlebar that looks better than the built in one with black text on a dark blue-gray background.
In my case I’ve made a template that I can drop on any new popup windows that has properties for setting the titlebar text and icon and also has a close button. I still have to add the custom properties (mX and mY) to the window’s root container.