Enable notification pages for HTTPS sites
In order for notification pages to be displayed for HTTPS sites - for example, block pages if the website is in a category that is blocked, or the Pre-logon welcome page for authentication - you must configure a root certificate on each client machine. This acts as a Certificate Authority for secure requests to the cloud proxy.
The setting is found on the page, under Settings. To enable it, mark the checkbox Use certificate to serve notifications for HTTPS pages.
This page also has a link to download the Forcepoint root certificate, which should be installed on client machines. For further details, see Forcepoint Web Security Cloud Help - Configure Block & Notification Pages.