class SiteController extends Controller
{
  // ...
  public function actions()
  {
    return [
      // ...
      'captcha' => [
        'class' => 'yii\captcha\CaptchaAction',
        'fixedVerifyCode' => YII_ENV_TEST ? 'testme' : null,
      ],
    ];
  }
  // ...
}
?>

class ContactForm extends Model
{
  // ...
  public $verifyCode;
  // ...
  public function rules()
  {
    return [
      // ...
      ['verifyCode', 'captcha'],
    ];
  }
  // ...
}
?>

$form = ActiveForm::begin(['id' => 'contact-form']); ?>
// ...
$form->field($model, 'verifyCode')->widget(Captcha::className()) ?>
// ...
ActiveForm::end(); ?>