In the MDrivenDesigner System prototyper, it is possible to add an expression to simulate login. This will be useful if your logic checks the current user—the prototyping does not identify 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