Discount Code code required for PayPal minicart (using Serif) - repost

Im creating a shop in Serif and am using the PayPal minicart option. I want my customer to be able to enter a discount code at checkout, but the cart doesn't allow this. If it's possible I would like some code that I can add to my site that will allow this. Please note that I don't want the discount on specific items - I want the client to be able to put in a code at checkout that will give a discount on the whole amount. The code must work in Serif The code must be editable, so that I can change what code is typed in to get the discount Payment will not be paid until the code is seen working, so please do not reply unless you can send me an example of this working