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.
After taking a look through Contact Form 7’s documentation, I wasn’t able to find a feature that would allow for securely storing passwords. You may want to check with Contact Form 7’s support team in case this type of functionality exists, however, I would not recommend passing this type of sensitive information over to Constant Contact, as there isn’t a secure method for that field to be passed over to Constant Contact or to be stored within a Constant Contact account.
Please have a look and let us know if you have any other questions!
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