The SpyFu API relies on a special API-only user name and password to log in, or you can use a Base 64 Key.
Log into SpyFu and go to your Account Settings>API Usage. You will see your General API info to help you set up your credentials.
The API docs page will ask for your username and password.
Use your SpyFu API ID as your username. Just below that is your secret key. Click to reveal that, and copy and paste that into the password field. Remember, both of these are available in your API Info inside Account Settings.
Base64 Option
Additionally, you can find the Base64 string has been pre-generated in your Account Settings under Base64 Key.
This is a different option available to you. This encoded string is sent in the "Authorization" header prefixed with the keyword "Basic." It is available for your use, but the user name and password method will work best for most people.