getSourceFieldOptions example

if ($allowed_type && !$field->isBaseField()) {
        $options[$field_name] = $field->getLabel();
      }
    }
    return $options;
  }

  /** * {@inheritdoc} */
  public function buildConfigurationForm(array $form, FormStateInterface $form_state) {
    $options = $this->getSourceFieldOptions();
    $form['source_field'] = [
      '#type' => 'select',
      '#title' => $this->t('Field with source information'),
      '#default_value' => $this->configuration['source_field'],
      '#empty_option' => $this->t('- Create -'),
      '#options' => $options,
      '#description' => $this->t('Select the field that will store essential information about the media item. If "Create" is selected a new field will be automatically created.'),
    ];

    if (!$options && $form_state->get('operation') === 'add') {
      $form['source_field']['#access'] = FALSE;