I've gone to Settings Services URL Shortening Custom. What do I enter as the URL shortening endpoint?
How can I use Goo.gl as my URL shortener in Twitter for iPhone (Tweetie)?
twitter;iphone;url shortening;goo.gl
At the moment you can't as there is no official API available. An extract from this post here:Even without an official API, there  are extensions  available for browsers  like Chrome (eg: goo.gl URL Shortener,  Shareaholic for Google Chrome) and  Firefox (eg: goo.gl lite). Before   people start writing code to  incorporate our new features, we  wanted  to let you know we do plan to  release an official API for goo.gl in   the future. Youll be able to use the  API to shorten URLs, expand  URLs, and  view analytics from directly within  your own applications