What is JotForm?
JotForm is a free online form builder which helps you create online forms without writing a single line of code. No sign-up required.
At JotForm, we want to make sure that you’re getting the online form builder help that you need. Our friendly customer support team is available 24/7.
We believe that if one user has a question, there could be more users who may have the same question. This is why many of our support forum threads are public and available to be searched and viewed. If you’d like help immediately, feel free to search for a similar question, or submit your question or concern.
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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.
Feel free to contact us if you need further assistance.