Fivetran does not support the private app flow. Currently the only options to authenticate the connector through the API are:

• Via the a connect card
• Creating your own public app within Hubspot and then supplying those Oauth credentials (client_id, client_secret and refresh_token) to Fivetran.

It was recommened by the Hubspot support team that we should use the private app flow for this which is:
https://developers.hubspot.com/docs/api/private-apps#make-api-calls-with-your-app-s-access-token