How to bind to someone else's ajax request?

Question

AJAX

The bottom line is, there is a site, I have access only to editing content, I can insert js into it, I need to somehow bind to the ajax request of the form on the site (this functionality is in the js file to which I do not have access), the question is - how to do it, in particular, I just need to find out what response the ajax server gave to the request from the form, 200 or not.
Thank you in advance for your help.