This is what I am using, and it works. Using Civil 3D 2024.
RESULT="--"
On Error Resume Next
Set oApp=GetObject(, "AutoCAD.Application")
Set oCivilApp=oApp.GetInterfaceObject("AeccXUiPipe.AeccPipeApplication.13.6")
Set obj=oCivilApp.ActiveDocument.HandleToObject("[Handle]")
RESULT=obj.StartStructure.Name
This only difference should be the version. (2023 uses 13.5) (2024 uses 13.6)