In the MDrivenDesigner System prototyper there is a possibility to add an expression to simulate login. This will come in handy if your logic checks the current user - the prototyping does not identifiy the current user and requests no login.

Consider this expression in the "Exp to run on start" box:

SysSingleton.oclSingleton.CurrentUser := SysUser.allinstances->select(username='someusername')->first