Identify Customers that Used a Discount Code
In Klaviyo, while there isn't a specific metric for discount codes, we will sync a Discount Codes property for each order where a discount was applied.
To identify all customers that have purchased using a specific discount code, you can use Klaviyo's Segment Builder tool.
Using the Segment Builder, you will want to use the condition:
What someone has done (or not done) >
has Placed Order at least once over all time >
where Discount Codes contains _____