I think, You are looking for an automatic way of populating your User name & Password whenever you are trying to login to the AS/400 System.
For this, You can Record a Macro and save it on your computer.
When ever you open the Navigator, You can play the Recorded Macro. It will populate the User Name, Password & other Recorded Commands as well.
Let me know if you need any clarifications on this or if you need any other info related to your requirement.