How to add a pop-up as soon as employee logs on to the portal 7.4?

We are using SAP portal 7.4.and Webdynpro ABAP. Our requirement is to add a pop-up as soon as the employee logs on to the portal and also we need to add a pop-up when the employee logs off from the portal.

I did look at the SDN threads and talking about AFP Masthead customization. I am not sure where to look for the Masthead. We are using Webdynpro ABAP. Can you please let me know the step by step process to add pop-up ?

Thanks

Sujata Kolla