curl --request PATCH \
  --url https://api.withflex.com/v1/refunds/{refund_id} \
  --header 'Authorization: Bearer <token>' \
  --header 'Content-Type: application/json' \
  --data '{
  "refund": {
    "reason": "duplicate",
    "metadata": {}
  }
}'

{
  "refund": {
    "refund_id": "<string>",
    "payment_intent_id": "<string>",
    "amount": 123,
    "created_at": "<string>",
    "reason": "duplicate",
    "status": "pending",
    "test_mode": true,
    "metadata": {},
    "reference_id": "<string>",
    "reference_type": "acquirer_reference_number",
    "reference_status": "available"
  }
}