The answer to http://meta.askubuntu.com/questions/3489/ is actually still correct, but Stack Exchange has changed their login page some so it isn't quite clear.  

On the login page you need to click "more login options" and then input your launchpad URL in the OpenID field. 

[![enter image description here][1]][3] ![enter image description here[2]][2]


  [1]: https://i.sstatic.net/2mdb4m.png
  [2]: https://i.sstatic.net/6Oa3sm.png
  [3]: https://i.sstatic.net/2mdb4.png