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.
If you're building your own integration/form, here are some of the endpoints that you can call in regards to contacts:
Bulk Activity Endpoint for Importing Contacts: https://v3.developer.constantcontact.com/api_guide/import_contacts.html
Endpoint for Creating or Updating a Contact https://v3.developer.constantcontact.com/api_guide/contacts_create_or_update.html
If needed, you may be able to find a front-end form in the OpenCart Extension Store: https://www.opencart.com/index.php?route=marketplace/extension
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