question

I assume you want to reuse a customized object from one model file in a different model. You need to export your customized object into a user library file. This file can you load into a new model file manually or always for any new model by environment settings. In a case where you need a set of different objects you put all customized objects into a single user library and export this.
Adding a user library by code makes only sense, if you are absolutely sure this action takes only place once during existence of your model or you need structures which deletes and unloads user library objects on each simulation run start.
https://docs.flexsim.com/en/23.2/Using3DObjects/CreatingCustomObjectsAnimations/CreatingCustomLibraries/CreatingCustomLibraries.html