How to add menu in model tree?

So not clear in which Customization module - so supposing it is a Toolkit question.

So a good point is to  check the PTC examples if there is some solution.

For example in the pt_user guide there is such menu item defined for part mode:

So means first you need to start the ptc examples / - here it is only the ptc user guide

So the next question is how to find the sourece of the desired function.

We see here the menu text is  -ExportXML.  So search for this text / string in the Toolkit installation directory

So there I found the UgPopupmenus.c as top point of the implementation (from there we need to check the further implementations of the subroutines and help libraries

You can additionaly try to find the implementation of the following menu items (in picture)