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I am not quite sure if I understood your concern correctly. Would you like to add your OWN signature on the form? If so, then you need to create that signature as an image using some 3rd party apps, or an online web app like this one: http://signature-maker.net/. After that, you could just add it as an image to your form.
If you wish to collect signatures from your users, then you can use our widgets related to electronic signatures.
You can check the following widgets:
I personally recommend using the Adobe DC eSign signature widget. Here's a guide on how to add widgets on your forms: https://www.jotform.com/help/252-How-to-Add-a-Widget-to-your-Form
Let us know if that helps.