How to get what the user want to print on printer?

I bought a desktop application called New Point, at the end of this app it prints the invoice,

here is the question could I get this invoice to my app before it gets printed in the printer so I can modify it, and if I could read the content of the invoice, using JS

you can’t find the app in GitHub or another to get the source code and change it, you have to buy this app, so I don’t think that you could get the source code