Drag and Drop with perspective camera 3D


I want to drag and drop 3d objectives in 3D Space with a perspective camera (55° top down).
Is there a chance to get the objective always in the same hight in 3d space over a terrain?

Just use a Plane to raycast with a ray from the camera, if you are doing an RTS.
or you can always shoot a ray from the camera Camera.ScreenPointToRay and get a point with a desired length r.GetPoint