Short answer
HubSpot's KB: under Send an email after form submission, click Create this workflow. Free tools allow one simple workflow per form with one action. Marketing Hub Starter allows up to 10 actions in a simple workflow.
1. In HubSpot
The relevant screen in my demo portal.

2. Worth knowing
Sending submissions to an outside address is a separate notification setting.
3. Related
Form follow-up email and submission notifications.