You can include the URL of the current page via defaultContent (As you
suggest).
I think using JS to get the current page would to be like this
<script type="text/javascript">
twttr.anywhere(function (T) {
T("#tbox").tweetBox({
height: 100,
width: 400,
defaultContent: function(){
window.location.href
}
});
});
</script>
Other than the tweet box, I've no idea where you would place the URL
of the page.