Please complete the following:
- You must go to Google developers console and create a new project (set fields "Project name" and "Project ID" as you wish).
- After creating the project go to the “APIs & auth” tab.
- Find in the list of avaliable apis “URL Shortener API” and set it ON.
- After that go to “Credentials” tab and click “CREATE NEW KEY” button in “Public API access” area.
- After all in pop-up window choose “browser key”. The generated key will appear on the page, and you can regenerate it anytime.