I know of a 3rd party plugin for Gravity Forms that can connect to constant contact. See https://wordpress.org/plugins/gravity-forms-constant-contact/
Please note that both of these are 3rd party products and not built or supported by Constant Contact directly, so I'm limited in the support I can provide with using them.
You can also create a landing page signup form within Constant Contact that will let subscribers check off which list(s) they want to sign up for. See this link for details:
Regards, David B. Tier II API Support Engineer
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