How can I have field data validation through an existing Google Spreadsheet?

Hi, I would like to validate one of the inputs in my form against an existing google spreadsheet, whereby suppose the input does not match any of the values in the google spreadsheet, the form would show an error.

While it is not possible to achieve what you kindly described through the form builder, you may consider introducing these changes by altering the full source code of your form. We have a guide on how to get the full source code of your form here: https://www.jotform.com/help/104-How-to-get-the-Full-Source-Code-of-your-Form

I hope this helps. If you need further assistance please let us know. Thank you.