This is a test of the new Tumblr editing features. It was easy to add that link by just selecting the text. I use the browser “Post to Tumblr” extension, though, and I don’t see the new editing features in that yet.
In this screenshot, you see the new editing bar behind the “Post on Tumblr” window where text is underlined, but no handy editing bar shows up. Since I like to post to Tumblr from my desktop and use the handy browser quick post method, the new editing feature won’t be to useful. Too bad.