Mailgun\Messages\MessageBuilder::addCampaignId PHP Method

    public function addCampaignId($campaignId)
    {
        if ($this->counters['attributes']['campaign_id'] < Api::CAMPAIGN_ID_LIMIT) {
            if (isset($this->message['o:campaign'])) {
                array_push($this->message['o:campaign'], $campaignId);
            } else {
                $this->message['o:campaign'] = [$campaignId];
            }
            $this->counters['attributes']['campaign_id'] += 1;
            return $this->message['o:campaign'];
        } else {
            throw new TooManyParameters(ExceptionMessages::TOO_MANY_PARAMETERS_CAMPAIGNS);
        }
    }

 /**
  * Add Mailgun campaign ID(s) to the message
  * Campaign ID limit is 3.
  *
  * @param int|string|array $campaigns
  *
  * @return \Bogardo\Mailgun\Mail\Message
  */
 public function campaign($campaigns)
 {
     $campaigns = array_slice((array) $campaigns, 0, 3);
     foreach ($campaigns as $campaign) {
         $this->messageBuilder->addCampaignId($campaign);
     }
     return $this;
 }