After you are logged in and added the items to the order list.

1. Click on the 'Total' 

2. Then click on 'Discount'

3a. To apply the default discount, click on the 'Select' spinner and select the discount & then click on the '✓' button

How to Create and Configure Discounts )

3b. To apply a discount manually, enter the Discount value, and Discount code & then click on the '✓' button

4. The discount will be applied

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