Launch Portal from Sapgui

Hello portal-experts!

I would like to launch the portal from SapGui, for example: Transaction ZPORTAL should open a browser window, automatically log on the user and display the portal.

I have configures SSO between the Portal and the ERP system and I know how to open a browser window (with the portal URL) but how can I enable automatic logon ? Or is there a more sophisticated solution ?

best regards,

Johannes