We do not offer a URL shortening feature.
URL shortening was required long ago because of Twitter's 140 character limit back then. They've changed their system so that any URL now occupies only 23 characters of text, regardless of the URL length, and they've increased their tweet length to 280 characters.
If you still want to shorten your URLs, the best is to open a new browser tab for bitly.com where you shorten the link, and then paste it into the post creation form text box.