curl --request POST \
  --url https://api.hubapi.com/marketing/marketing-events/2026-03/events/{externalEventId}/{subscriberState}/email-upsert \
  --header 'Authorization: Bearer <token>' \
  --header 'Content-Type: application/json' \
  --data '
{
  "inputs": [
    {
      "contactProperties": {},
      "email": "<string>",
      "interactionDateTime": 123,
      "properties": {}
    }
  ]
}
'

{
  "message": "Invalid input (details will vary based on the error)",
  "correlationId": "aeb5f871-7f07-4993-9211-075dc63e7cbf",
  "category": "VALIDATION_ERROR",
  "links": {
    "knowledge-base": "https://www.hubspot.com/products/service/knowledge-base"
  }
}