ExampleConfigEntityTwoForm.php

<?php

namespace Drupal\ctools_wizard_test\Form;

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

/**
 * Simple wizard step form.
 */
class ExampleConfigEntityTwoForm extends FormBase {
    
    /**
     * {@inheritdoc}
     */
    public function getFormId() {
        return 'ctools_wizard_test_config_entity_two_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['two'] = [
            '#title' => $this->t('Two'),
            '#type' => 'textfield',
            '#default_value' => $config_entity->getTwo() ?: '',
        ];
        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('two', $form_state->getValue('two'));
    }

}