Welcome to the Community, @Anonymous.
You can add the discount as an invoice item with a negative amount. It will automatically calculate the total once it's added to the invoice.

When you categorize it, you can record it as an expense under Business in the Transactions menu.
I'd also recommend reaching out to an accountant about this. They can give you options on how you can categorize the discount in QBSE.
I'm also adding here some great articles that can help you with categorizing your transactions and on how to navigate QBSE with its functions:
If there's anything else I can assist you with, let me know. I'll get back to you as fast as I can. Have a safe day!