Currently I'm using this in my code:
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.
It looks like we've already followed up with you via email, but here is the information provided for visibility on the forum:
I'm sorry but at this time our V3 API requires OAuth 2.0 to authenticate the customer's account and there is no way to bypass it.