How can I start my forms with "start filing" button?

It's a Smart PDF Form. You would need to import a PDF form into Jotform to have that feature.

Please refer to this page for more details: https://www.jotform.com/help/605-how-to-use-and-get-the-best-out-of-jotform-smart-pdf-forms

This is not applicable to a regular Jotform form.