How to take customer name from POS(online) erpnext v12 and pass as input to a python file?

Hi folks,

I want to fetch the customer name input from POS(online) erpnext v12 and pass to a python function in custom app created by me. How can I do that?

Purpose: To fetch customer details associated with the entered customer like available loyalty points.

Right now, I created one .py and one .js file with same name in custom app.
Before posting here, I went through https://github.com/frappe/frappe/wiki/Client-Side-Scripting-Index link.
All I learnt is to make js/frappe calls to python files

My intention: How to take customer name from POS(online) erpnext v12 and pass as input to a python file?

Any Help.
Thank You.