I want to add email address to a particular contact list. How do i authenticate to that customers account details to add the email address ?
The Constant Contact API authenticates to each user's account by using an OAuth 2 access token. If you are building an integration that is specifically for the use of one specific user, it is possible to easily generate an access token for that account using the mashery I/O Docs. It is important to note that one of the steps in authenticating an account with the API is to log in to Constant Contact with the desired account's credentials.
If you are looking to allow many users to connect with Constant Contact through your integration, then it may be best to implement a full OAuth 2 flow for generating access tokens. If you will be following this path, you can find a document regarding this flow here: http://developer.constantcontact.com/docs/developer-guides/authentication.html