action – Always subscribe.
ACCOUNT_ID
account_login
CAMPAIGN_ID
campaign_name
CONTACT_ID
contact_name (optional)
contact_email
contact_ip (optional) – In case of double-optin this is the IP than confirmation link was clicked from.
contact_origin – One of import, email, www, panel, leads, sale, api, forward, survey, iphone, copy, landing_page.