how to get asp.net hidden fields values with node js?

i am trying to login in a site with nodejs. the asp.net login form has hidden fields. like __EVENTTARGET, __EVENTVALILDATION, __VIEWSTATE, __EVENTARGUMENT i am trying the following code

process.env['NODE_TLS_REJECT_UNAUTHORIZED'] = '0'; // Ignore 'UNABLE_TO_VERIFY_LEAF_SIGNATURE' authorization error

var request = require('request');
var cheerio = require('cheerio');

function postback(url, callback) {
    request.get(url, function onResponse(err, res, body) {
             $ = cheerio.load(body);

        var button = callback(err, $);
                        if (button) {
            var form = {
                //__EVENTTARGET: button.attr('form1').match(/['"]([^"^']+)/)[0], // TODO image buttons
                __EVENTTARGET: button.attr('Button1'),
                __VIEWSTATE: $('#__VIEWSTATE').val(),
                __EVENTVALIDATION: $('#__EVENTVALIDATION').val(),
                __EVENTARGUMENT: $('__EVENTARGUMENT').val(),
                        };
                }
    });
}



postback('https://example.com', function(err, $) {
 var button=$('input[type="image"]');
return button
  //console.log(err)
//console.log($)


})

its not getting the values of __EVENTTARGET and __EVENTARGUMENT . these fields may needs a button click.. how to click it with cheerio?