Inventor model tree structure

[dhanshri_gavas]

I have an assembly document open. In that assembly document, I've added a subassembly (using the default template). Now, within that subassembly, I want to import a STEP file or add another subassembly. However, currently, the imported part or subassembly is added to the main assembly. How can I add a part or assembly inside the subassembly using the API?
now it is looking like this

I want structure like this

Thanks in advance!

[WCrihfield]

If you want to insert sub assemblies or imported models under a sub assembly, then you must open that sub assembly directly, then insert them directly within that document.  You can not do it remotely from a higher level assembly.