Thank you for reaching out to the API Support team here at Constant Contact.
When working with our current V2 API, the two options you have for action_by when submitting a contact are:
As long as your integration is for a contact signing themselves up, you can use ACTION_BY_VISITOR. If you are not, you may consider instead creating an Autoresponder email campaign that is sent to all contacts who are new to a list, regardless of how they are added.
Please let me know if you have any other questions!
API Support Engineer