How to define position from assy to prt

[Scux]

If I want to use Default as Constrain type in Component Placement, how do I set up the geometric link between assy and part? Is it possible to place a plane, point or similar in the assy that will show up in the part to be able to align the part with the assy?
I have been struggling with this for a while now, it might be something simple is miss here but I just dont get it...

/Daniel