Actually on this site that is not the AddThis control in the header, I'm using the older separate controls and for the FacebookLikeButton I do have the UrlToLike hard coded as the url for our facebook fan page. The other buttons are using the current page url as is the AddThisButton in the blog using the post url.
But the same principle applies, you can hard code a value for the UrlToShare property on the AddThisWidget.
Hope that helps,
Joe