Unfortunately, this method that you described is not possible for me to follow because this option is NOT available for me in Quickbooks. I have attached a screenshot for your reference. I had already tried doing it before asking this question in the community.
That is why I put is as a purchase discount in a negative amount, in the bill itself. But since you said we cannot add discounts in the bill, and your described method is not possible either, therefore please suggest another alternative to go about for this.
The Discount option is only available in the Pay bills window. If you want to enter it on the Bills window, you can continue the steps you're doing by entering the discount as a negative amount. Then, you can enter a description of the amount that it's a discount. Review the AmountDue before saving and closing it. You can also click Recalculate from the menu if you made changes to the amounts.
Meanwhile, you can change the affected account of the discount (pay bill option) through Preferences. Let me show you how;
Click the Edit menu and select Preferences.
From the left menu, click Bills.
Select Company Preferences.
Click the preferred Default Discount Account.
QuickBooks, don't retroact so I suggest that you toggle the discount item again. I'd also recommend reaching out to an accountant of there is another way of applying discounts to the bill. This way, we won't mess up your books. You can also use this article to know more about our Accounts Payable workflows in QuickBooks Desktop.
If there's anything else that you need help with, just leave a reply below. You take care always and have a great day!