Florian:
In the Object's 3D Script you will need to add an
AddZ command with the value of 1/2 the height of the Object (whether that is a fixed value or a parameter), probably at the top of the script.
You should add a Signature to your Profile (click the Profile button near the top of this page) with your ArchiCAD version and operating system (see mine for an example) for more accurate help in this forum.
David