Short answer
HubSpot's KB: once the object is defined, create records manually, by import, or with a workflow Create record action. In workflows, the custom object needs a defined relationship with the workflow's object. Records can also sync from Salesforce.
1. In HubSpot
The relevant screen in my demo portal.

2. Worth knowing
Decide whether you need a custom object at all before building one.
3. Related
When to build a custom object.