WooCommerce - Add new Custom Fields to Emails

functions.php

add_filter('woocommerce_email_order_meta_keys', 'xa_email_custom_order_meta_keys');
function xa_email_custom_order_meta_keys( $field_keys ) {
     // Please make sure the Custom Field 'tracking_code' is added to the Checkout Field.
     $field_keys[] = 'tracking_code'; 
     return $field_keys;
}