To create a Point on Poly constraint, you select a target mesh or a specific target point on that mesh, then select the object you want to constrain to that target.
To create a Point on Poly constraint
To ensure the object is constrained in the position and orientation you expect, make any necessary adjustments to its pivot point before you constraint it to the mesh. (See The pivot point and Change the pivot point in the Basics guide.)
If you selected an entire mesh as the target, the constrained object moves to the origin when you create the constraint. You can then adjust its position relative to the target mesh by editing the U and V parameters in the constraint attributes. See Edit point on poly constraint attributes.
If you selected a specific vertex, face, or edge as the target, the constrained object is placed at the precise UV co-ordinates of that point when you create the constraint.