How can I find out which button was clicked to submit the form?

Question

JavaScript

The site has 4 buttons. Clicking on them opens the same form.
How to indicate in a letter by clicking on which button a pop-up window was opened and the form was sent?
The option to make different shapes for each button is not suitable.

Answer 1

When clicking, write to a secret (hidden) field?

Answer 2

hidden input write to it which button was pressed

Answer 3

This is the basic functionality of html forms.
Without registration, without SMS, and scripts.
You can test here: https://jsfiddle.net/z8555bpw/