Coupons are always applied to the Income Account that you have configured in your Xero Settings page. For example:
In this case ^, the coupon would always be applied to the "Other Revenue" Income Account that is configured.
We don't have a way to edit the Income Account specifically for coupon discounts, but you can manually edit the Income Account it is applied to via the Invoice Edit page in Xero after the sync has already occurred.
To do this, go to the Invoices page in Xero -> Lookup the invoice that had a coupon applied -> Click Edit -> Click on the box under the Account column next to the coupon line item and a drop down menu will appear where you can change the Account it is applied to:
I hope this is helpful, but please contact our Support team if you have any questions - firstname.lastname@example.org