Thank you for reaching out to Constant Contact API Developer Support.
Unfortunately our API can only be called from the back end of a website. There are no current plans to add support for cross-origin resource sharing, so you would need to use a server-side process to make the actual calls to our API.
Solutions that others have used is to have a separate server and pass data via AJAX to that separate server, which then makes the API calls on your server behind the scenes.
Please let me know if you have any questions!
Regards, David B. Tier II API Support Engineer
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