To update the instance created by the defer node your workflow needs to create either an update or a completion trigger. Messages from the different types of triggers can be displayed in your portal page.
The processing entry points are the same as a Connector.
The figure below is a common configuration that will cause the Incident number to display on the portal page as well as information when the Incident is closed. The completion trigger can pass variables like resolving group/individual, resolution notes and more. These variables can be referenced in the Complete Message field which will cause the information to be visible on the portal page.
The Remedy form that stores the message is:
You can search the form by the Instance GUID which relates to the instance Id field on the KS_TSK_Instance form.