Executing Chrome extension onclick instead of page load

I created a Chrome extension that works as expected except that it only executes when I load a page that matches the conditions in the manifest. I have tried for hours to make it execute by clicking on the extension icon to no avail.

The closest I have been able to what I want is that I have been able to make the extension icon click to run the code, but then it does not run it on the loaded page. It runs it on the extension's space instead of the page DOM.

In the current state my code only runs when a page in the specified domain opens. I want to run it only when it matches that rule, but only when I click the extension icon.

Here is my code: manifest.json

    {
        "name": "Get Response URL",
        "version": "1.0",
        "manifest_version": 2,
        "browser_action": {
        "default_icon": "mkto_icon.png",
        "name": "Click to get URL"
        },
        "content_scripts": [{
            "js": ["contentscript.js"],
            "matches": ["http://mydomain.com/*"]
        }]
    }

contentscript.js

    if (document.getElementsByName("returnURL")){
        alert("\nThe Response URL on this form is:\n\n" + document.getElementsByName("returnURL")[0].value);
    }