MyWorks Sync supports syncing orders containing discounts/coupon codes into Xero, utilizing the Discount column within the Xero invoice or quote.

Syncing Discounts to the Xero "Discount" column

By default, MyWorks Sync will sync any discounts in a WooCommerce order to the Xero column field for Discounts.

This would be shown in the Xero order in the following example.