I'm more familiar with MailChimp and their API - over there, a list can be configured to require a field (like "FNAME" for first name), and if a list is so configured, then all API calls to add list subscribers MUST include both the email address AND the first name.
Is this configuration possible at Constant Contact? If so, can you point me to the API documentation that describes this configuration? Or, does creating a new contact and adding them to an existing list ONLY require the email address, and NEVER require any other fields?
I can definitely help you with the questions that you have about our API. When using our API, you do not specify the requirements of the contacts in the lists. When creating a contact the only thing that is required is an email address and a list for the contact to be added.
If you have any further questions please do not hesitate to ask!