Look at your original form and copy one of the form fields, changing any names or ID's etc. Or add a perfectly standard HTML form element to the form.
The form is generated from a view, the views are in the views folder. The view is called from a controller usually which you will find in the controllers folder. The controller is usually called via the url with domain_name/controller/method or you might have to check the routes file in case they are using routes to match a url to a controller.
Then when you have your additional field, go to the controller that manages the page and find the form validation rule set. Copy one of the rule sets to an additional one and change the name and the rules.
Then in the controller (or model or library) that manages the input, do whatever you need for it.
CI is a light framework that is super easy to learn. Perhaps you could spend a little while learning it. If they are saying this is a proof of concept, then usually that means it is shoddy short cut code that is not implemented properly. I would be extremely wary of using it in a production environment.
