You will need to get a goo.gl API Key to use this feature.
How to get a key
Visit the Google APIs
Console, and:
- Create a project. You can create as many or as few
projects as you need. (See the Google APIs Console FAQ for
details.) Google will generate exactly one key per project.
- Activate the URL Shortener API. After creating a project,
you should see a list of APIs, each with an Activate button. Click the one for the url shortener.
- Nab the key. Click "Keys" on the left-hand side to find
out about yours. The "Value" is the string you want.
- Copy and Paste the Key in the Goo.gl API Key field and save the changes.