While trying to help a friend troubleshoot why sharing a blog article only shares her homepage (and not the article), I noticed that canonical URL's aren't generated for blog posts. AddThis support is saying this is what their service uses to pass the URL for sharing.

SO, they have an option to add a variable for the URL to explicitly set the URL being shared, but I can't find a way to get the URL of the blog post into the tag. Here's the required code:
<div class="addthis_toolbox addthis_default_style" addthis:url="THE URL OF YOUR PAGE">
Does anyone know how to 1) generate canonical URL's for the blog page, or 2) pull the current page URL into the variable? I've tried [url] and [product_url] and neither work. Thanks!