Hello user7941713,   Thank you for reaching out to Constant Contact API Developer Support. My team is here to assist outside software developers with questions about building into Constant Contact's API.   You can definitely build your own sign-up form that’s capable of adding submitted contacts to a specific list using Constant Contact’s V3 API. Constant Contact provides developers with free access to our V3 API endpoints for developers building their own application for integration. You can get started using our API by visiting our documentation here: https://v3.developer.constantcontact.com/api_guide/getting_started.html    Additionally, you can utilize our API Reference Page to find body schemas & values, as well as test the functionality of our available endpoints to see how they will work once programmed within your application: https://v3.developer.constantcontact.com/api_reference/index.html   Instructions for generating a V3 API key and Client Secret can be found in steps 1-3 at the following link.: https://v3.developer.constantcontact.com/api_guide/apps_create.html   Below I’m including documentation for our contact creation and update endpoints that you could utilize for your form.   POST (create) a Contact: https://developer.constantcontact.com/api_reference/index.html#tag/Contacts/operation/createContact   PUT (update) a Contact: https://developer.constantcontact.com/api_reference/index.html#tag/Contacts/operation/putContact   Create or Update a Contact: https://developer.constantcontact.com/api_reference/index.html#tag/Contacts/operation/createOrUpdateContact   The request for V3 application code samples is currently under review. However, we're currently still adding endpoints and capabilities to the V3 API, and do not plan to publish official sample apps until that process is complete. Your feedback and experience with this request is essential to improving our product, so thank you for reaching out to us regarding this matter.   For updates on which new features and endpoints have been released for Constant Contact's V3 API, you can stay updated by checking the Constant Contact V3 API Release Notes page:   V3 API - Release Notes https://developer.constantcontact.com/api_guide/index.html   In the meantime, here are the example calls that are currently available for V3:   V3 Code Samples: https://v3.developer.constantcontact.com/api_guide/tag_code_samples.html    Additionally, you can utilize our API Reference tester to test out the functionality of our available endpoints to see how they will work once programmed within your application: https://v3.developer.constantcontact.com/api_reference/index.html    Please have a look and let us know if you have any other questions!   Regards, ... View more