code: private function getAdmJson(): string
      {
          $adm = [
              'data' => $this->getTrimmedJson([$this, 'getAdmJsonInner'], static::ADM_MAX_LENGTH, 'You message for ADM is too long'),
              'expiresAfter' => $this->ttl,
          ];
  
          foreach ($adm['data'] as $key => $value) {
              if (!\is_string($value)) {
                  $adm['data']["{$key}_json"] = json_encode($value);
                  unset($adm['data'][$key]);
              }
          }
  
          if ($this->collapseKey != static::NO_COLLAPSE) {
              $adm['consolidationKey'] = $this->collapseKey;
          }
  
          return json_encode($adm, JSON_UNESCAPED_UNICODE);
      }
docs: Get the json to send via Amazon Device Messaging.