Click on the customer you wish to add the discount for
Click on subscription and then click on the subscription you want to add the discount to. The list of subscription products will appear on the right-hand side
click on the advanced overrides drop down menu, select discounts and click GO
A pop up will appear like the one below. Click ADD DISCOUNT
After clicking ADD DISCOUNT, another pop up will appear like the one below. Fill in the fields and click add.
The added discount will appear in the grid as shown below and then click APPLY to save the discount to the subscription product
The discount will automatically be included in the invoice once the above is done.