How to calculate the anchor point location, when 2 meshes join by a hinge joint

I have 2 rigidbody meshes. when they touch each other I want to attach them to each other with a hinge joint. The hinge joint should be set at the location they touch.

The problem is how to get the anchor point value for the hinge joint. I know the contact points from Collision.

Can anyone tell me how to calculate the anchor point?