If this maps to options in a check list or radio button list you do the following



- In survey editor go to your question

- Find the response you want the text box added to

- Click that response option and you should see a drop down arrow appear

- Click the arrow and select the option "Allow Text Entry"

- When you click the arrow again you will see additional options to control the size and validation of content in that box



If this isn't what you are hoping for, I would then suggest adding a text entry question and using display logic so the proper people are asked your follow up question.

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