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.
Unfortunately it is not possible to prepopulate signature field.
Possible workaround would be to add image of you signature to your form. I have cloned your form and added image with signature to my cloned form.
Here is my cloned form: https://form.jotform.com/60472318433957
You can also add signature to your form after your user submit the form and then send your user Autoresponder email with both signatures. Inside of this thread my colleague provided step by step instructions how this can be done http://www.jotform.com/answers/404783
Hope this will help. Let us know if you need further assistance.