let newWindow = open('https://javascript.info', 'example', 'width=300,height=300');
newWindow.onload=()=>{
alert("Popup Loaded");
}
Onload event was not triggering after the popup window is loaded.
Since it's not the same domain , you will not be able to access the window set properties.
Try this here in console
(function(ow) {
ow.addEventListener("load", function() { alert("loaded"); }, false);
})(window.open('https://stackoverflow.com/', 'example', 'width=300,height=300'));
This will work
But if you put a different domain , it will not work. You'd have to use PostMessage or something else.