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Currently the way that we allow contacts to be added into our system without the use of our Join My Mailing List form, which is on our site, would be to use our API's. Our API's have two forms of authentication that must be used.
The first form of authentication is OAUTH. With OAUTH, the developer does not need to know the password of the client; however, the client would need to initially put in their password on our side, which then stores an access token on your side, but does not store the password with you. You can find more about OAUTH here.
The second form of authentication is basic authentication, which does require the clients username, password, and also a valid API key which you can obtain from here.
If you have any questions regarding this please let me know.
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