Keyboard string, textbox always shows up

Hi. I am doing an experiment where I use Likert scales for the first 19 questions and the last question I present is open-ended. If I make the participant input for the open-ended question as a “keyboard-string”, then the textbox will appear on the first 19 questions as well. Is there any way I can have the textbox appear only on the last question and not on any of the other questions?

This link explains how it is done:

If you are still having trouble, please post your experiment as an Experiment Package. This is found under the File menu within SuperLab.

Okay, thank you.