how are you accessing the Authenticate User behavior?
it sounds like you are trying to create a new authenticate user behavior by going to WebAssist -> Security Assist -> Authenticate User instead of editing the existing one on the login page.
to edit the existing behavior, go to the Server Behaviors panel (Window -> Server Behaviors) and double click the authenticate user server behavior in the list.
If you set the option to Remember My User Name and Automatically Log in when running the wizard, there will be 2 behaviors, you will need to edit both.
In the archived Documentation section of the security assist support page, there is a tutorial for User Level authentication that covers this for Security Assist 1, it is the same process for Security Assist 2.