here is a working html example.
but i think you should make a web site (home page) at google sites (its free) and
open a browser from the game via button.
i put in a company url www.btnet.de at link as test referer.
if you are not signed in(i think the browser use cookies) a new window with google login appears.
<html>
<head>
<title>+1 Basic</title>
<link rel="canonical" href="http://www.btnet.de" />
<script type="text/javascript" src="https://apis.google.com/js/plusone.js">
</script>
</head>
<body>
<g:plusone></g:plusone>
</body>
</html>
