// First you"ll need this
$BossBarAPI = $this->getServer()->getPluginManager()->getPlugin("BossBarAPI");

// You can also do:

use PocketEssential\BossBarAPI\BossBarAPI;

// Set the barMessage first!

$message = "This is a BossBar message";

$BossBarAPI->setBarMessage($message);

/*
 Player should an instance of a PLAYER, Just foreach loop all the online players; etc
 */

$BossBarAPI->sendBossBar($player);


  }