for example, the confirmed subscription hook posts this data to your server.
"type": "subscribe",
"fired_at": "2009-03-26 21:35:57",
"data[id]": "8a25ff1d98",
"data[list_id]": "a6b5da1054",
"data[email]": "blahblahblah",
"data[email_type]": "html",
"data[merges][EMAIL]": "blahblahblahATblah.com",
"data[merges][FNAME]": "MailChimp",
"data[merges][LNAME]": "API",
"data[merges][INTERESTS]": "Group1,Group2",
"data[ip_opt]": "10.20.10.30",
"data[ip_signup]": "10.20.10.30"
And setting custom thank you pages can be done with their application.