Hi, @marko82.
Thanks for posting in the Community. I'd be glad to help you add a discount to an invoice.
Before doing so, we'll need to turn on first the Discount feature to easily add and track discounts in your invoices.
Here's how to turn on:
1. Go to the Gear icon and then Account and Settings.
2. Select Sales.
3. In the Sales form content section, select Edit.
4. Turn on the Discount setting.
5. Select Save and then Done.
Once done, you can now add a discount in your invoice. Follow the steps below.
1. Click the +New button and select Invoice.
2. Fill out the form and add your products or services in the Product/Service column.
3. To record a discount, select Discount percent to enter the discount as a percentage. Or select Discount value to enter the discount as a specific dollar amount.
4. Enter the percentage or amount you want to discount in the discount field.
5. Select Save.
Please refer to the screenshot below for your reference.

You can also add a discount as an item of your inventory. Check out this article for the process: Add a discount as a line item on your sales forms.
Let me know if you have further questions about adding a discount to an invoice or any other QuickBooks-related issues. I'm always around to help. Keep safe and have a nice day!