Try this :
Get the 3D Point from these commands (and don't move the camera after this) :
WantedX = Get3DVectorXFromScreen(50,50)
WantedY = Get3DVectorYFromScreen(50,50)
WantedZ = Get3DVectorZFromScreen(50,50)
(you may need to add camera coordinates if not at 0,0,0, as explained is the example in the documentation..)
And multiply the WantedZ coord by the distance (for the wanted size on screen) you want and apply WantedX , WantedY, WantedZ to your object.
Documentation with example :
https://www.appgamekit.com/documentation/Reference/3D/Get3DVectorXFromScreen.htm