In the subscription_created event,  the coupon Id can be found as the entity_Id under the invoice >> discounts. Here's a sample for your reference. 

"discounts": [{
            "object": "discount",
            "entity_type": "document_level_coupon",
            "description": "$50 off",
            "amount": 5000,
            "entity_id": "$50OFF"
        }]