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.
I think you're looking for a look-up function which is currently not available in Jotform. I have requested that to our widget team.
For now, you may consider using this widget instead: http://widgets.jotform.com/widget/autocomp. You have to add the existing entries to the option of this widget.