code: public function withValidator(Validator $validator)
      {
          // Determine if the HMAC is correct
          $validator->after(function (Validator $validator) {
              if (!$this->request->has('code')) {
                  // No code, continue
                  return;
              }
  
              // Determine if the HMAC is correct
              $shop = ShopifyApp::shop($this->request->get('shop'));
              $authHandler = new AuthShopHandler($shop);
  
              if (!$authHandler->verifyRequest($this->request->all())) {
                  $validator->errors()->add('signature', 'Not a valid signature.');
              }
          });
      }
docs: Configure the validator instance.
  
  @param \Illuminate\Validation\Validator $validator
  
  @return void