I'm using X Theme and Cornerstone. The button is displayed within each post. I've since decided to place the html in a content template and add the template as raw data, but it's still not showing correctly, even with the changes you suggested.
OK, I just tested this myself locally to see what's missing, and what's missing is specifying raw output for your URL field.
The Types URL field will output a link tag by default, not just the URL saved in the database. You can tell it to output just the saved URL by adding the output attribute like so: