<?php $form = ActiveForm::begin([
  'id'   => $model->formName(),
  'action' => ['/apitools/default/index']
]); ?>

<?php
$js = <<<JS
// get the form id and set the event
$('form#{$model->formName()}').on('beforeSubmit', function(e) {
  var \$form = $(this);
  // do whatever here, see the parameter \$form? is a jQuery Element to your form
}).on('submit', function(e){
  e.preventDefault();
});
JS;
$this->registerJs($js);

<?php JsBlock::begin() ?>
  <script>
    $(function () {
      jQuery('form#apitool').on('beforeSubmit', function (e) {
        var $form = $(this);
        $.ajax({
          url: $form.attr('action'),
          type: 'post',
          data: $form.serialize(),
          success: function (data) {
            // do something
          }
        });
      }).on('submit', function (e) {
        e.preventDefault();
      });
  </script>
<?php JsBlock::end() ?>