How to add an extra field to comment.php

Today I’ve searched the whole internet for how to add an extra field to the comments.php file which is visible in a comment in my admin. I have a comments.php file with a name, e-mail and textfield. But I want to add an extra field for location.

There are lots of tutorials for adjusting the default WordPress comment_form but I am searching for a piece of code for a custom made comment form.

If you can give me a step in the right direction, let me know in the comments please.