From the User Name menu at the top of the page, select WebAPI.
Default WebAPI screen
Click Create WebAPI Key.
If you ever delete your API key, you can always return to this page and create a new one. (You cannot continue to use any previous keys that have been deleted.)
Checking Your API Keys
Once you have finished creating an API key, your WebAPI status will change to "activated"; you will also see your user ID and the generated (API Auth) key.
If you no longer wish to use the web API, click Disable WebAPI to delete your key.