Not too sure what you mean. Do you want to create a new case when a new portal is opened? What's the business use case for that? This will prevent users from using the portal to examine or review existing cases assigned to him without creating a new case.

I have a similar requirement to create a case when the portal is opened through SSO. I also need to open the case. I have used a javascript onload event to create the case and passed the assignment handle to the open assignment method.