SessionTestForm.php

<?php

namespace Drupal\session_test\Form;

use Drupal\Component\Render\FormattableMarkup;
use Drupal\Core\Form\FormBase;
use Drupal\Core\Form\FormStateInterface;

/**
 * Form controller for the test config edit forms.
 *
 * @internal
 */
class SessionTestForm extends FormBase {
    
    /**
     * {@inheritdoc}
     */
    public function getFormId() {
        return 'session_test_form';
    }
    
    /**
     * {@inheritdoc}
     */
    public function buildForm(array $form, FormStateInterface $form_state) {
        $form['input'] = [
            '#type' => 'textfield',
            '#title' => 'Input',
            '#required' => TRUE,
        ];
        $form['actions'] = [
            '#type' => 'actions',
        ];
        $form['actions']['submit'] = [
            '#type' => 'submit',
            '#value' => 'Save',
        ];
        return $form;
    }
    
    /**
     * {@inheritdoc}
     */
    public function submitForm(array &$form, FormStateInterface $form_state) {
        $this->messenger()
            ->addStatus(new FormattableMarkup('Ok: @input', [
            '@input' => $form_state->getValue('input'),
        ]));
    }

}