[chimerax-users] Hopefully simple plugin development question

Hello, I am looking at writing a plugin for chimerax that takes a pdb and runs some python code that will calc some stuff hand stuff to other programs. I know how to do this. What I want to know is how to find the current model's name / associated pdb filename from my plugin. Imagine my plugin really is just a "Run" button that looks up the current model, generates the associated pdb filename, and passes that name to my python script. Can someone tell me how in a chimerax plugin I can make the call that will give me the pdb filename I want? I've read much of the developer docs and could not find what I was looking for.