Custom tab doesn't automatically logs in

The customer has set up SSO integration with the WebEx app and on-premise CUCM for calling integration.

This works, they login to WebEx with SSO, and the CUCM calling is auto-logged in and registered.

They added a custom tab to a website that is using the same SSO IdP as the WebEx client. They want that the custom tab is auto-logged in as the phone services. The idea would be that it would use the token from WebEx/CUCM to also login to this custom tab application. This does not seem to work as they are presented with the IdP login page when browsing to the custom tab.