Docs
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”: [{
Articles & FAQ "object": "discount",
"entity_type": "document_level_coupon",
"description": "$50 off",
"amount": 5000,
**"entity_id": "$50OFF"**
}]
