How to pre-populate the form data from another form

May we know what kind form are you trying to do so we can assist you properly? You can also provide more details of what you're trying to achieve. 

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I want customers to fill out a number of forms which request information but only have to put in their personal details once on a "sign in" type form. When they go to each form the personal details will be automatically pre-populated from the sign in 

Thank you for providing the additional details.

You can pass the data from one form to another by using URL parameters method. Our user guide "How to Automatically Pass Form Data to Another Form" explains the detailed process on how to achieve this. The article lists all the steps to achieve what you require.

As you mentioned Sign-in forms, please be informed that you cannot use fields like login and password as it's considered as a phishing. You can refer to our terms for more information https://www.jotform.com/terms/

I hope this helps and get in touch with us if you need further information.