I believe that chrome does not allow the popups to open an external page by any way. The only solution I know is to place an iframe in your popup.html file with src attribute set to popup2.html and place all your html inside popup2.html. However, consider that all websites do not work well inside iframe.

If you are trying to open url in currently active tab then try following:

Attach following script to your popup.html file:

var hrefs = document.getElementsByTagName("a");

function openLink() {
    var href = this.href;
    chrome.tabs.query({active: true, currentWindow: true}, function(tabs) {
        var tab = tabs[0];
        chrome.tabs.update(tab.id, {url: href});
    });
}

for (var i=0,a; a=hrefs[i]; ++i) {
    hrefs[i].addEventListener('click', openLink);
}

You need to add tabs permisson to your manifest file for this to work.

There is a 3 step workaround you can do this by

• You should know when the popup will be opened. Popups are blocked in chrome by default. But chrome will show a popup blocked notification in bottom right corner.
• click on notification to select always accept popup from this specific site
• Reload the main (parent) page. This will load the popup in small windows. Right click in titlebar of popup window and select "Show As Tab". Once popup is transformed in a window with tab, you can drag it back to main window.

Hope this helps.

Thanks.