Which element does the ifc_opening_id need to be written to?

Dear community,

I try to write an ifc-file, converted from a dwg file. Simply extruding a block and using the extrude for IfcWall, IfcOpeningElement and IfcDoor. (I am going to add the attributes later). Now the only thing that I am missing is the ifcrelfillselement and ifcrelvoidselement. I followed the instructions given by the explanation on the FME page "Industry Foundation Class STEP Files (IFC) Feature Representation" and the hints given by @dominik​ concerning this issue.

But I am not quite sure, where to add the ifc_opening_id and what values to assign to it. Could someone help me out here?

Thanks a lot in advance,

Morten