Edit registration form php

I need to add a lengthy terms and conditions scrolling text box to the registration form. This is a for a free form hooked up to mailchimp.

I saw a tutorial on a pop up with terms and conditions. I don’t want this. I want a scrolling text box before the Sign up button.

I know I can add custom php or html to the bottom of the form. But that would look bad because it would be after the signup button.

Is it safe to add some html for the scrolling text box before the submit button on the php file?

What is the name of this php file and where is it located?

Best!

https://wordpress.org/plugins/s2member/