Getting discount coupon value from checkout page

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I am trying to get the value of discount code from the checkout page (I don't have Shopify Plus).
The discount code might not be valid (and it can be 2-3 letters) I still want to sent it to my server through some webhook or API. Is that possible what route would you suggest? (I checked the webhook that checks when a user adds some valid code but I want the code also when it isn't valid "discounts/redeemcode_added" )
Thanks !