Thanks for posting here in the Community, Yunii.
I'm here to share some insights on how the markdown option works in QuickBooks Desktop.
One way to add a markdown in QuickBooks is to add a Discount item on your sales transaction.
To do so, please follow the steps below:
To add a discount item:
- Select Lists at the top, then choose Item List.
- Click the Item drop-down arrow and select New.
- In the New Item window, click the Type drop-down arrow and then click Discount.
- Enter an item name, such as Discount.
- Enter the Description that you want QuickBooks to put on your sales forms when you apply the discount.
- In the Amount or % field, enter the discount amount or percentage:
- If the discount is a percentage, enter the number of the discount followed by the % symbol.
- If your discount amounts vary, you may want to leave the Amount or % field blank and enter the amount directly on your sales forms.
- Click the Account drop-down arrow and then click the income account you want to use to track discounts you give to customers.
- When an income account tracks discounts on sales, the account is often called a "contra-income" account.
- Click the Tax Code drop-down arrow and choose a tax code for this item.
- Click OK or click Next to create another item (see screenshot below).

To add the discount item on a transaction:
- Open a sales transaction.
- Under the item section, insert the discount item immediately after the item that you want to discount.
- Enter the needed details and click Save and close.

To learn more about adding a discount item, visit this article: Add, edit, and delete items.
That should work for you. If you have other questions about QuickBooks, feel free to let me know. I'm always happy to help. Have a good day!