Short answer
HubSpot's KB: the trigger enrolls records when a webhook is received from a third-party app. Only application/json is supported, the selected webhook property must be non-empty, and the workflow only triggers if an existing record matches a unique property value. You can create up to 100 unique webhook events.
1. The trigger card
Trigger settings in my demo portal.
2. Creating records
The trigger doesn't create records. Create them through the API or an integration first.