In order to use the Integrations Platform APIs (currently this includes the Timeline API and Webhook API), you'll need to sign up for a HubSpot developer account and create a new application. If you don't already have a developer account, you can sign up for an account directly at the developers signup page.
For most use cases, we assume that only OAuth'd apps have access to user portals. That implies that objects created with these APIs (like Timeline Events) won't show on user portals until your app is OAuth'd on their portals.
While you're setting up your Timeline Events or Webhooks (or future APIs) you can use the developer HAPIKey, found at the bottom of your Developer Dashboard.
The Integration Platform APIs use a different authentication type for setup. In addition to being able to set up the integration through the UI from the details for your app, you can also set up the the integration using your Developer HAPIkey, which you can find on the Apps page inside your developer portal. In addition to being used to set up your apps, the Developer HAPIKey will also let you make HubSpot API calls on your development portal without requiring OAuth.
As an example for the Timeline API, you would use the Developer HAPIkey to create the event type, and set up the event properties. After the event is set up, you would use the access_token you'd get from the OAuth flow to create an event on the timeline of a contact.
Once you have a developer account and an application created and setup, you'll need to implement our OAuth flow in order for HubSpot users to grant your app access to their portals. See our OAuth documentation for how to do this.