Pass GET parameter to buttons

Hi, I’m trying to pass a dynamic GET parameter to the output of the Button shortcode.

For this, I need to locate where the Button code is in the theme folder and modify it a bit to accept a parameter, and append it to the link output by the button. I can’t find the code for the buttons though, can you point me to it please?

Or do you have a filter for the button?

Thanks,
Tom