There are two ways to add a custom discount. They are as follows.
Open up Shopify Admin and click on Discounts from the navigation. Here you can see a green button to create new discounts.
Clicking on that will open up a popup with the different types of discounts that you can create. Click on any of the products under Custom Checkout & Discounts.
Open the Custom Checkout and Discounts app and navigate to Advanced Discounts in the sidebar. Click on any of the discount types available above. If you would like to add a regular Shopify discount, then click on any of the discounts listed under Legacy