ExampleConfigEntityOneForm.php

<?php

namespace Drupal\ctools_wizard_test\Form;

use Drupal\Core\Form\FormBase;
use Drupal\Core\Form\FormStateInterface;
use Drupal\Core\Url;

/**
 * Simple wizard step form.
 */
class ExampleConfigEntityOneForm extends FormBase {
    
    /**
     * {@inheritdoc}
     */
    public function getFormId() {
        return 'ctools_wizard_test_config_entity_one_form';
    }
    
    /**
     * {@inheritdoc}
     */
    public function buildForm(array $form, FormStateInterface $form_state) {
        $cached_values = $form_state->getTemporaryValue('wizard');
        
        /** @var \Drupal\ctools_wizard_test\Entity\ExampleConfigEntity $page */
        $config_entity = $cached_values['ctools_wizard_test_config_entity'];
        $form['one'] = [
            '#title' => $this->t('One'),
            '#type' => 'textfield',
            '#default_value' => $config_entity->getOne() ?: '',
        ];
        $form['external'] = [
            '#type' => 'link',
            '#title' => $this->t('Show on dialog'),
            '#url' => new Url('entity.ctools_wizard_test_config_entity.external_form', [
                'machine_name' => $config_entity->id(),
            ]),
            '#attributes' => [
                'class' => 'use-ajax',
                'data-dialog-type' => 'modal',
            ],
        ];
        return $form;
    }
    
    /**
     * {@inheritdoc}
     */
    public function submitForm(array &$form, FormStateInterface $form_state) {
        $cached_values = $form_state->getTemporaryValue('wizard');
        
        /** @var \Drupal\ctools_wizard_test\Entity\ExampleConfigEntity $page */
        $config_entity = $cached_values['ctools_wizard_test_config_entity'];
        $config_entity->set('one', $form_state->getValue('one'));
    }

}