How to edit php code in WordPress Post file?

Question

I have a wordpress page. In that wordpress page, user will have access to the following Window to Publish their Post in to the webpage.

If the user enters an URL in any of the three filed I have marked in red above, my php code will look in to those URLs and determine if they are spam or not. Therefore, I want to put my php code inside the Publish button, so I can either disallow or allow the post after my php code run and determine if those URLs are spam or not.

Can you please tell me the name of the wordpress file I should edit in order to put my php code in. I am new to wordpress and don’t know too much about their file structure.