Short answer
HubSpot's KB: if the form fails or the embed code changes on the live page, insert it with a plugin. Extra CSS may be needed for responsiveness. If non-HubSpot form data doesn't arrive, another plugin may conflict. The article covers legacy forms only.
1. In HubSpot
The relevant screen in my demo portal.

2. Worth knowing
Check the KB for current options for forms built in the updated editor.