You can create a discount from the EG rule page that matches the rule's conditions. The reward can be discounted with an EG automatic discount, a Shopify automatic discount or a Shopify promo code.
Set the trigger, action, schedule and targeting first. The discount is built from these settings.
In the Discounts section of the rule page, pick a method and click Create new.

Choose which other discounts this one can combine with. Each of those other discounts also has to be set to combine with this one, or they won't stack. See Combining discounts in Shopify. Click Continue when you're done.

Save the rule, then test it on your store to confirm it works the way you intended.