[otrs] Need help to create a custom field

Hello, Anyone can tell me how to add a custom field on submitting a new ticket ? I would add a new menu field on the page CustomerTicketMessage and insert that field on the database. I tried to find where insertion is done on the pm files, but I didn't undestand how this is done. I give an example on new ticket page I have 3 fields To : Subject : Text : I would to have this To : Ref : Subject: Text: When ticket is submitted all fields are inserted into the database and extracted later. thank you for help