Apply Header Level Coupon after all other Discounts

We need to setup a Coupon Modifier to be applied after all other Line Modifiers are applied to the Order.

But we are not able to define Bucket for a Coupon Modifier ?

Basically we want to apply the Coupon Discount after the all other standard discounts.

So we thought if we can define a new bucket for Coupon .. but the Bucket field is not editable for Coupon Modifier.