Laminar flow module in Comsol 4.3a

Hi, everyone, I am using laminar flow module in Comsol 4.3a to model Navier-Stokes equation. My question is how I should add the body force term F into the equation? The term F is listed in the equation of laminar flow module(shown in the attached picture), but I cannot find any place to input the expression for F on the interface.
Thanks a lot and any help would be greatly appreciated!