Freshbooks Classic uses basic authentication. For this, you will need your Authentication Token.
Finding your authentication token
You can find your authentication token by:
Going to your freshbooks page (https://[yoursubdomain].freshbooks.com
Clicking "Profile" at the top
Scrolling down to the "API Access" section.
Authenticating the connector
You can then authenticate your connector by entering your Authentication Token in the "Username" field, and a single letter X in the Password field.
Your subdomain is whatever comes before freshbooks in your freshbooks web address. So if your address is http://johnsmith.freshbooks.com , the subdomain will be johnsmith