Simply add field an existing form how to please

I had a little app dumped on me that was created using Codeignighter.  After I got the App working on the server they tell me "This was just a proof of concept now we need it jump through hoops o-flame and do back flips off the high dive into a shot glass."

After light coding with PHP for 15 years I have no idea what is going on in Codeignighter code. 

All I want to know for now is to add one field to a form that exists and have it processed just like the fields already in the form. Which successfully write a database.

How please & simple on my dumb old frustrated thinky brain.
Thanks