The Streak API is tied to your Streak account. Simply login into Gmail and make sure Streak is enabled. Click on the Streak dropdown in the top right navigation bar of Gmail and find the "Developers" button.
Click on the button for Developers.
This will open a dialog where you can view your current API key, delete it or create a new one. Treat your API Key like a password as it has full access to your account. If you feel it has been compromised, you can always regenerate a new one from the dialog.
Keep your API key(s) secret.