Two fields mandatory to match

This post introduces how to make that as an example:

Custom validation

Where would you put the code?

Hello @danhughes,

You can put the code in the functions.php file of an active child theme or within a custom plugin.

Note that you can also use the filter hooks in the functions.php file of a parent theme, but if this theme is updated you will lose the changes.

Please read Child Theme > How to Create a Child Theme to learn more. If you want to go further, take a look at How to Build a Custom Functionality Plugin for WordPress.

Best regards,
Yordan.