I just saw your message, and I hope it's not too late or maybe help others in the future.
For macOS exports (but I believe the Windows procedure is quite similar):
1. Export your game/app;
2. Right-click the *.app file → Show Package Contents
3. Navigate to Contents/Resources/app.nw/index.html
4. Open the index.html file with a code editor (Sublime Text, Atom, etc);
5. After the <body> tag, add a <div> tag just like shown below:
<div style="left:108px;width:490px;height:70px;position:absolute;-webkit-app-region:drag;">
</div>
Use "left, width, height" arguments to define the position and size of your draggable area.