Make the text box a separate control on the page, and then check in your code line if another is checked. If so, pull the value of the text box and use it.
To answer the question in your editing: You will need to play with the CSS page in order to position it correctly. How you do this depends, in particular, on the layout of the page. I recommend posting part of the HTML from your page in another question and asking how to place them.
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