Tumblr has a sharer just like Facebook and Twitter, Tumblr's is: http://www.tumblr.com/share/. I believe that Tumblr's works by looking at the referring URL and uses that.
I cannot seem to find any documentation to override that. I want to pass in an additional key into the URL that is sent by default. Any ideas how to do this?
Answer
The URL format is as follows:
http://tumblr.com/share?s=&v=3&t=[Title]&u=[uri encoded URL]
This will pre-populate the share form with the chosen title and url. As noted the URL, or value of u= must be URI encoded to work properly.