Hubspot Marketing - Why GET /timeline-event-types returns a 400 response

The "/timeline-event-types" resource has specific requirements from the Hubspot API that a Developer API Key (hapikey) be passed on requests and not an OAuth access token, while other resources require an OAuth token. To use this resource when authenticating via OAuth you must supply the "HubSpot API Key" in the Optional Fields section when provisioning your instance of Hubspot Marketing, and you also must authenticate against your own Hubspot OAuth Application. The OAuth App and the Developer API Key must be generated in the same Hubspot account to successfully interact with the endpoints of the "timeline-event-types" resource.