function login() {
  var url = "http://...";
  var username = document.getElementById("username").value;
  var password = document.getElementById("password").value;

  var xhr = new XMLHttpRequest();
  xhr.open("POST", url, true);

  xhr.setRequestHeader("Content-Type", "application/json");

  xhr.onreadystatechange = function () {
    if (this.readyState == 4 && this.status == 200) {
      alert(xhr.responseText);
      window.open(welcome);
    }
  };

  var obj = { UserName: username , Password: password }
  var data = JSON.stringify(obj);

  xhr.send(data);
}