Edit in JSFiddle

document.addEventListener("DOMContentLoaded", function() {

  function changeFn() {
    var values = form.get();

    if (values.login && values.password) {
      form.buttons[1].enable();
    } else {
      form.buttons[1].disable();
    }
  }

  var form = new FancyForm({
    renderTo: 'container',
    title: 'Login',
    width: 290,
    height: 250,
    items: [{
      label: 'Login',
      type: 'string',
      name: 'login',
      emptyText: 'Username or email address',
      events: [{
        change: changeFn
      }]
    }, {
      label: 'Password',
      type: 'password',
      emptyText: 'Password',
      name: 'password',
      showPassTip: true,
      events: [{
        change: changeFn
      }]
    }, {
      inputLabel: 'Remember me?',
      type: 'checkbox',
      name: 'remember',
      value: true
    }],
    buttons: ['side', {
      text: 'Clear',
      handler: function() {
        this.clear();
      }
    }, {
      text: 'Login',
      handler: function() {

      }
    }]
  });

});

<script src="https://cdn.fancygrid.com/fancy.min.js"></script>

<div id="container"></div>