//popup.html

<html>
    <body>
        <input type = "button" id = "the_button" value = "My button" onclick = "sayHi()"></input>
    </body>
    <script src = "popup.js" ></script>
</html>

//popup.js

document.getElementById("the_button").addEventListener("click", function() {
    var win = window.open("http://www.roblox.com", "roblox", 400, 400)
    //this is where I'm having some problems
    win.ready(function() {
        console.log(win.location.href)
    })
})

//manifest.json

{
  "manifest_version": 2,
  "name": "Test",
  "description": "Test Extension",
  "version": "1.0",

  "icons": {
    "48": "icon.png"
   },

  "permissions": [
    "http://*/*",
    "https://*/*"
  ],

  "content_scripts": [{
    "matches": ["http://*/*", "http://*/*"],
    "js": ["jquery.js", "popup.js"]
  }],

  "browser_action": {
    "default_title": "This is a test",
    "default_icon": "icon.png",
    "default_popup": "popup.html"
  }
}