How can I link 2 blueprints (skeletal mesh and static mesh) with "Physics Constraint Actor"?

Hello guys, I have a problem about linking a skeletal mesh and a static mesh (cart) by using Physics Constraint Actor.
Firstly, I put a cube(static mesh) inside the skeletal mesh blueprint. Then I linked these blueprints refer to cube (which is inside the skeletal mesh) and cart. But cart is moving into the center of skeletal mesh and ignored the cube (even i refer it).

I tried also linking in the same blueprint. But problem is still continue as you can see at link given below.

I’m looking for the exact same feature for days . Anyone know how to do this ?