HTML

<form-builder id="builder"></form-builder>
<script>
  document.querySelector('#builder').controlList = [{ control: 'textbox' }, { control: 'combobox' }, { control: 'document' }];
  document.querySelector('#builder').addEventListener('form-builder-updated', (event) => {
    console.log('form-builder-updated ', event.detail);
  });
</script>

Demo