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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.