Due to limitations in Constant Contact's API, MailMunch is not able to sync certain form fields (e.g. Birthday and Anniversary fields) with Constant Contact which are otherwise available in Constant Contact's UI.
Constant Contact's API does not have an endpoint for third-party services like MailMunch to fetch user's custom fields. Therefore, if you've created a custom field in Constant Contact, you will not be able to map that against any form field in MailMunch.
Our developers keep a constant check on any updates announced for Constant Contact's API. Rest assured, we'll implement these changes as soon as Constant Contact makes them available in their API. For more information, please visit https://developer.constantcontact.com/docs/contacts-api/api-gui-func.html