Hi , Based on my research, currently, Microsoft Defender for Endpoint does not support the customization of the default webpage that is shown to a user on a device that has been isolated. The default webpage is designed to inform the user that their device has been isolated and prevent them from navigating to other webpages.
You could, however, use the Microsoft Defender for Endpoint APIs and power automate to create a custom solution that notifies users when their device has been isolated, potentially providing them with a link to a custom webpage.