Form to show message based on user input + no submit button

Hi there, how do I create a form where when a user submits a unique code, they are shown a particular message. For instance, when a user enter "12345", they are shown "Message 1". If a user enters a different code "789012", they are shown "Message 2". Also, I want the form to not show any Submit button. 
Can you please suggest how do I achieve this?