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If you are trying to add the underline to the label in your field, you can do it by adding the next CSS code in the Designer tool :
This guide will help you to find the field's ID : http://www.jotform.com/help/146-How-to-find-Field-IDs-Names
And if you are trying to add a underline when using the Text, just follow the image below :
Hope this helps.
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