Apply Position, rotation, scale of null object to single x,y point param

[AnmolM]

I have an an effect that has an entry for Source - which is a point x,y param. The layer has been tracked and the motion applied to null object "Null 1"

I have keyframes on Position, rotation, scale for "Null 1" 

What expression will capture the transform for Position, Rotation, Scale and apply it to the x,y co-ordinates of the Source - point parameter ?

[Jose Panadero]

What you can do is to link (not parenting) the point XY parameter of your effect directly to the null's position.

[AnmolM]

Link - not parenting. Like this ? - https://helpx.adobe.com/in/after-effects/how-to/animating-with-parenting.html 

I thought parenting was only done between layers. Is it possible to parent a single param to a layer ?

[Meng Zhiqun]

Behinds the parent pickwhip method, if you open the dropdown for each of the transform properties, like position, you can see that each property has its own pickwhip, just like on the layer.

To link properties easily, you can click drag this pick whip to another property of another layer. Take note that this is not the parent pickwhip, but the pickwhip of the property itself.

After that, you can see that an expression will be generated. This is how to "parent" properties, instead of the layers itself. Hope this helps!