ServiceNow has provided the below KB article for options on enabling the CSP for Portal page at page level.
https://support.servicenow.com/kb?id=kb_article_view&sysparm_article=KB2947482
By OOB. CSP can be added to a portal page via HTTP response header. As anyone added CSP at page level using the below option provided by the KB article
Option 3: Scripted Conditional Header via Business Rule or Scripted REST API (Recommended for Granular Control) Implement a custom Business Rule or Scripted REST API that programmatically sets the CSP header based on the requested URL path. This allows the header to be applied only where iFrame embedding is required. Note: this is a custom implementation and falls outside the scope of out-of-box platform support.
