We have built a custom Post URL sign up form and want to customize the error message/link. Right now it goes to visitor2.constantcontact.com/api/signup.
You can test it here: http://www.cedarsinaipk.dreamhosters.com/shelly-testing-page/
Thank you for reaching out to Constant Contact API Support.
The response from the submission will come back with a status and a responseText JSON object. If you are using a custom signup form that uses our inline embedded signup endpoint, you might have success using something similar to the jQuery example on our Tech Blog post about that endpoint: http://techblog.constantcontact.com/api/release-updates/announcing-a-new-signup-form-post-url/
Please let me know if you have any other questions!
Tier II API Support Engineer