When inserting a full-length link, you will use up more characters. But if you check Enable short links, it will automatically turn it into a short link taking up much less characters. It requires that you have a full http:// or https:// link to utilize.
The character count will assume your link is now 22 characters long, so you can keep typing your message with confidence that you're using the maximum 138 characters that remain once your link is shortened.
How to Use Short Links
Go to the Campaigns tab.
Click on the Create New Campaign button.
Scroll down to the message box, and check the Shorten Links box.
Note: You can check the box before or after entering the link in your message box.