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.
We don't have dedicated developer accounts per se, but you are able to create a free trial account with limited sending capabilities which you can use 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