You have two options to define water table in domain 1. Phreatic line 2. Steady state. I suggest you to see manual and liquifaction example to make sure which is most suitable.
See, seepage/outflow are related to water head mostly in case of embankment dam, coffer dams its used. I do not think you need to use it for liquefaction, you need to define a water table location in your domain.
This paper has modelled a well know case history of Down-hole arrays at the Wildlife site using plaxis UBC sand model in plaxis,