Solved it : https://myriad.website/create-popup-window/
here is working : https://myriad.website/learn-python/
First install the “Blank Slate” plugin.
Create New Page
Leave the Title empty
Set in Page property Template=”Blank Slate”
Add whatever content you want
Because there is no Title, we need set a Permalink so that is human readable, let say : your-site/blah
Publish the page
Go to the page you want to have the popup window show up on clicking a link
Add the following code :
<a href="" onclick='window.open("https://your-site/blah/","window-name","width=400,height=320");' target="blank">Popup window</a>