PHP - trigger code based on submit action

Hi, I have a form where a user submits by a button. When that button is being clicked I want to run a piece of code. That's the task in its short form. I have a wordpress site, where a user reaches a landing page where the user can buy a product. In the URL of that landing page are 4 query parameters, that are being stored as cookies. When the user wants to purchase the products, the user enters their information in a pop-up form. When the user submits the form by clicking a button, I would like to run some code, that triggers a postback URL for server-2-server tracking. Knowledge of postback URL/server-2-server tracking is not needed. I have attached a picture of the piece of code that needs to run when the submit button of the form is clicked. So the task is: 1. Locate the submit button 2. Place an if function that runs the attached code when submit button is clicked 3. Check that cookies are being passed to the postback URL 4. Check that server response is active (I can check whether the server gets a response) The task could take only a few minutes or up to an hour.