Issue #3102358 by Phil Wolstenholme, eojthebrave: If the field is not being...

    $field = $form_state->getFormObject()->getEntity();

    $form['third_party_settings']['datalayer']['expose'] = [

      '#type' => 'checkbox',

      '#title' => t('Expose in dataLayer'),

      '#title' => t('Expose field data in JavaScript <code>dataLayer</code> variable.'),

      '#default_value' => $field->getThirdPartySetting('datalayer', 'expose', 0),

      '#description' => t('Checking this box will result in this field being included in the dataLayer object.'),

      '#description' => t("Checking this box will result in this field's value being included in the <code>dataLayer</code> object provided by the dataLayer module."),

    ];

    $form['third_party_settings']['datalayer']['label'] = [

      '#type' => 'textfield',

      '#title' => t('DataLayer label'),

      '#default_value' => $field->getThirdPartySetting('datalayer', 'label', $field->get('field_name')),

      '#description' => t('Enter the label you would like in the datalayer output.'),

      '#description' => t('Enter the label/key to use when adding this field to the <code>datalayer</code> object. Example; <code>dataLayer.fieldName: value</code>'),

      '#states' => [

        'visible' => [

          ':input[name="third_party_settings[datalayer][expose]"]' => [

            'checked' => TRUE,

          ],

        ],

      ],

    ];

  }