I've gone to Settings → Services → URL Shortening → Custom. What do I enter as the "URL shortening endpoint"?
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. You’ll be able to use the API to shorten URLs, expand URLs, and view analytics from directly within your own applications
Not sure if this works, but here is a PHP class that uses an unofficial Goo.gl API: http://code.basvd.nl/google_url_shortener_1.0/