How to use signature field in custom forms?

I have installed and enabled this module, but when I put the below code into a custom form it does nothing - I also noticed the library assets are not loaded, so I feel like it's not doing anything. Is there anything else I need to do to make this work?

<?php
    $form['signature_field'] = [
      '#type' => 'signature',
    ];
    $form['signature_data'] = [
      '#type' => 'hidden',
      '#attributes' => [
        'id' => ['signature_data'],
      ],
    ];
?>

I finally got this working using the code below. It seems like the selector isn't there in the code example, so wrapping in a <div class="signature"> was needed. Also, the signature_data element selector is wrong in the example, the id is signature-data and not signature_data

<?php
    $form['signature'] = [
      '#type' => 'container',
      '#prefix' => '<div class="signature">',
      '#suffix' => '</div>',
    ];
    $form['signature']['signature_field'] = [
      '#type' => 'signature',
    ];
    $form['signature']['signature_data'] = [
      '#type' => 'textarea',
      '#attributes' => [
        'id' => ['signature-data'],
        'class' => [
          'signature-data',
        ],
      ],
    ];
?>