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.
It looks like we've already followed up with you directly via email, but here is the information provided for visibility on the forum:
We don't have a developer account per say but you are able to create a free trial account through our developer portal which will allow you to have a trial account with limited sending capabilities. From there you can use the trial account for your testing purposes. Below is a link to our developer portal where you can sign up for a trial account, you'll just need to click on "Get Started" and then "Sign up".
Constant Contact Developer Portal
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