Form redirection on submit

Example : (Insert this line in to your form submitForm() methode)

Redirect to the user page ( Redirection on form submit)
$form_state->setRedirect('user.page');

Redirect to a custom page
$form_state->setRedirect('your.module.route.name');

Like:

  public function submitForm(array &$form, FormStateInterface $form_state) {
   $form_state->setRedirect('your.module.route.name');
  }

Using a Url Object (use Drupal\Core\Url)

$url = Url::fromRoute('route.name', [
  'arg1' => "value1",
  'arg1' => "value2",
]);
$form_state->setRedirectUrl($url);