To display a fixed promo code on the validation page of your coupon, go to Validation methods in the After Claim in the coupon builder and go to Show the coupon value.

Click on the dropdown menu underneath 'Title:', select Promo code and press Save.

Next to 'Title:', you are able to set up a fixed promo code under Discount value:. That promo code will then be displayed on the validation page of your coupon.