To post the form to the correct address you must combine page's base url and the one in the post attribute. url.resolve(from, to) can help
To post the form you can use e.g. http://visionmedia.github.io/superagent/ or some other ajax library with support for application/x-www-form-urlencoded
You should be aware that many sites apply various anti-spam measures and they will reject your requests if not properly formed. It must be checked on per-site basis.
You should also be aware that mass mailing or mass form submitting is in some countries prosecuted by law.
You should be aware that there are usually much easier APIs to be used by 3rd parties (other then parsing html forms) based on JSON and REST or SOAP.
If you need to submit pages to a web server legally better option would be to negotiate another API with the server owner. It should not be a problem even with government servers as many of them are opening due to the Open Data initiative (America, Europe, India, ..)