What to do in situations with loading DOM and scripts, provided that nothing in html can be changed?

Question

JavaScript

I have one application.
It has index/htm some code and one script loading (main.js).
The application is built in such a way that
I cannot change anything in index.htm.
The task, the task is to make the preloader while the main.js file is being loaded (6-7s)
But how to do this if the same preloader is written in the main.js itself.
Any ideas, work with gulp, webpack, all sorts of tricks are welcome.
Only one condition index.htm remains unchanged, everything else I can afford

Answer 1

Write only the preloader in main.js and load the next js from it.