Hello Mekafime,

Go to Coordinate System and Insert a Coordinate system. Pick the face and a coordinate system origin will be created at the center of that face. Use Hit Point Normal to align the Principal Axis with the face.

Then you can use that coordinate system in the definition of the motion of that face. 

Perhaps you want to apply a Displacement, then you can type a zero for X and Y and a value for Z in that coordinate system and it will take that face and only move it axially and not allow any other motion.