ComplexTestTypeStateForm.php

<?php

namespace Drupal\workflow_type_test\Form;

use Drupal\Core\Form\FormStateInterface;
use Drupal\workflows\Plugin\WorkflowTypeStateFormBase;

/**
 * Form to configure the complex test workflow states.
 *
 * @see \Drupal\workflow_type_test\Plugin\WorkflowType\ComplexTestType
 */
class ComplexTestTypeStateForm extends WorkflowTypeStateFormBase {
    
    /**
     * {@inheritdoc}
     */
    public function buildConfigurationForm(array $form, FormStateInterface $form_state) {
        $state = $form_state->get('state');
        $configuration = $this->workflowType
            ->getConfiguration();
        $form['extra'] = [
            '#type' => 'textfield',
            '#title' => $this->t('Extra'),
            '#description' => $this->t('Extra information added to state'),
            '#default_value' => $state && isset($configuration['states'][$state->id()]['extra']) ? $configuration['states'][$state->id()]['extra'] : '',
        ];
        return $form;
    }

}