It looks like we've already followed up with you via email, but here is the information provided for visibility on the forum:
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.
If all you need is a simple sign-up form for your website I would recommend using the standard Constant Contact WordPress Plugin. That way you won't have to sign-up for OptinMonster as well. In order to utilize the Constant Contact WordPress Sign-up Form Plugin you would need to add it to your website. Just in case, below is a link to documentation on the Constant Contact WordPress Plugin.
Install the Constant Contact Forms Plugin for WordPress to Gather Sign-Ups and Feedback: