Hello DanielleA843,
Thank you for reaching out to Constant Contact API Developer Support, and for your patience. My team is here to assist outside software developers with questions about building into Constant Contact's API.
With a standard List Join automation series, any method of adding a contact to a list will trigger the series to deploy (as mentioned above), but it does work differently in the case of Welcome Emails. While they do fall under the automation family, there are additional requirements.
In order to receive a Welcome email, the address must be new to the account (never added previously) and join with the source of “Contact” rather than “Account”. Contacts added manually via the UI would not be eligible for a Welcome Email.
The document you referenced notes that “contacts added using either of the two contact_import bulk activity endpoints have create_source=Account, and are not sent Welcome Emails”, but you can use the Add a Single Contact endpoint instead, which allows you to set the source as “Contact”, which in turn would make them eligible to receive a welcome email.
Add a Single Contact https://v3.developer.constantcontact.com/api_guide/contacts_create.html#add-a-single-contact
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