wanting printf to write to textbox buffer, but not overwrite

I have a chat box example that I need help with.

I have a chat box submit text to another disabled chat box in order to display
the chat. The problem is, when I use AGAR's printf function, it overwrites the
buffer, therefore deleting previous messages. How can I save previous messages
in the buffer and display the new ones as well?

Thanks in advance,
Chris