No code exists to support, and no method is known to exist, that provides the ability to add a banner to the Performance Management Log In page(s).
Closest ability is adding a custom logo to the pages post log in as documented here:
https://techdocs.broadcom.com/content/broadcom/techdocs/us/en/ca-enterprise-software/it-operations-management/performance-management/3-7/administrating/performance-center-administration/customize-a-theme.html
The ability to add a banner as described would be a new feature. This can be requested by posting an Idea request to the Performance Management Community site for consideration.