PayPal\Rest\ApiContext::addRequestHeader PHP Method

    public function addRequestHeader($name, $value)
    {
        // Determine if the name already has a 'http.headers' prefix. If not, add one.
        if (!(substr($name, 0, strlen('http.headers')) === 'http.headers')) {
            $name = 'http.headers.' . $name;
        }
        PayPalConfigManager::getInstance()->addConfigs(array($name => $value));
    }

 /**
  * Set api context
  *
  * @param $website
  * @return $this
  */
 public function setApiContext($website = null)
 {
     $this->_apiContext = new ApiContext(new OAuthTokenCredential(Mage::getStoreConfig('iways_paypalplus/api/client_id', $website), Mage::getStoreConfig('iways_paypalplus/api/client_secret', $website)));
     $this->_mode = Mage::getStoreConfig('iways_paypalplus/api/mode', $website);
     $this->_apiContext->setConfig(array('http.ConnectionTimeOut' => 30, 'http.Retry' => 1, 'mode' => $this->_mode, 'log.LogEnabled' => Mage::getStoreConfig('dev/log/active', $website), 'log.FileName' => Mage::getBaseDir('log') . DS . 'PayPal.log', 'log.LogLevel' => 'INFO'));
     $this->_apiContext->addRequestHeader('PayPal-Partner-Attribution-Id', 'Magento_Cart_PayPalPlus');
     return $this;
 }