Moving ground plane content to Image target´s position

I am trying to achieve the following: user places some AR content on a ground plane. When a Image target is detected, I want to reposition the AR content to the image target's position.

Simply setting the AR content´s position to image target´s position (which is always zero, as far as I know) does not work, because it somehow breaks the perspective - after that moving the camera around shows that the ARContent does not stay in place anymore when moving.

So clearly something more sophisticated calculation is needed - my question is: how can I get an image targets location in `"Ground plane space"?

Im using Unity 2018.2.5f1 with Vuforia 7.2.23.