Min lösning för detta var att använda en ZendDB-fråga istället för get-funktionen.
https://docs.directus.io/api/data.html #zend-db-tablegateway
'item.create.orders:before' => function (\Directus\Hook\Payload $payload) {
$customer_id = $payload->get('customer'); //get customer_id from 'order' item
$container = \Directus\Application\Application::getInstance()->getContainer();
$dbConnection = $container->get('database'); // connect to database
$table = new \Zend\Db\TableGateway\TableGateway('customers', $dbConnection); // connect to table
$results = $table->select(['id' => $customer_id]); // select row with 'customer_id'
$customer = $results->current(); // get array of current 'customer' row
$reseller = $customer->reseller; // get 'reseller' column value from row
$payload->set('reseller',$reseller); //Update Payload
return $payload;
}