RedirectBlockForm.php

<?php

namespace Drupal\form_test\Form;

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

/**
 * Builds a simple form that redirects on submit.
 *
 * @internal
 *
 * @see \Drupal\form_test\Plugin\Block\RedirectFormBlock
 */
class RedirectBlockForm extends FormBase {
    
    /**
     * {@inheritdoc}
     */
    public function getFormId() {
        return 'redirect_block_form';
    }
    
    /**
     * {@inheritdoc}
     */
    public function buildForm(array $form, FormStateInterface $form_state) {
        $form['actions'] = [
            '#type' => 'actions',
        ];
        $form['actions']['submit'] = [
            '#type' => 'submit',
            '#value' => $this->t('Submit'),
        ];
        return $form;
    }
    
    /**
     * {@inheritdoc}
     */
    public function submitForm(array &$form, FormStateInterface $form_state) {
        $form_state->setRedirect('form_test.route1', [], [
            'query' => [
                'test1' => 'test2',
            ],
        ]);
    }

}