Current State:

When adding a form to an issue, the default visibility is internal:

How forms behave on work items.

New forms added to work items are set to Internal by default, meaning that only agents and admins can see the form on the work item. Forms can be made external which also allows customers to see the form in the portal. 

Proposed Future State:

The ability to set the default visibility to be either internal or external.

Alternative (e.g. Third Party Plugin):

1. To add the form via automation rule and using the Attach form action. From there you can chance the form visibility.
2. To change the form visibility via REST API: Change Form visibility using Automation and API