I don’t have access to Hubspot so I can’t say for certain, but reviewing the Hubsmot Create a Broadcast Message API endpoint, the triggerAt field expects a date value.
| Trigger At | triggerAt: {timestamp} |
|---|---|
| used in the request body | A Unix timestamp in milliseconds of the time that the message should be published to the channel. If this is not included in the request, the message will be published immediately. |
Try passing, Unix Timestamp date value in the field, for testing you can pass formatDate(now;X) and see if that works for you.