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Hello, you will not integrate it with the form itself (within JotForm) because we do not allow insertion of any sort of script on any part/section of your form.
Your understanding is correct that you need to use the Full Source code of your form, but integrating your JS function would mean you would do it from your end. The general workflow would be as follows:
1. You grab your form's Full Source code.
2. You embed it anywhere you like.
3. On that same page where you embedded the full source code, that's where you need to point your JS function to validate the contents of a textbox on your form.
If I were in your shoes, I would approach this by hooking up your function to the element ID of the textbox.