How to add this action?

I want to call a function when click Submit comment botton. This function is to change some special text in the comments, and it’s only needed when post comments.

In WP documents, it says can write this way:
add_action ( ‘hook_name’, ‘your_function_name’,)

What should I write as “hook_name” ? Also, should I use wp’s original hook, or should I use the theme’s hook?

After add_action, should I write do_action ?

Thanks!