Current State:

Currently, JSM request Forms visibility can be restricted as documented here:
https://support.atlassian.com/jira-service-management-cloud/docs/restrict-who-can-view-forms/
Only users and groups with Atlassian accounts can be given access to restricted forms. Project roles, customer accounts, and customer organizations are not supported at this time.

There is no way to explicitly target only portal‑only customer accounts, and

Proposed Future State:

Introduce support for restricting JSM form visibility by customer account type, so project admins can:

Explicitly exclude:Portal‑only customer accounts
 

Alternative (e.g. Third Party Plugin):

NA