With the all searching so far I have found that we can add custom variable using this:
$objectManager = \Magento\Framework\App\ObjectManager::getInstance();
$model = $objectManager->get('Magento\Variable\Model\Variable')->loadByCode('custom-variable-code');
$plain_value = $model->getPlainValue();
but I have no idea where to add this and what to do with plain value. Please let me know how can I create custom variable which I can use in my email template globally. Thanks