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Auto-fill feature enable from form advance setting under form preferences. To setup Auto-fill feature, please follow these steps;
1) Click on "Preferences" button from form toolbar.
2) Click "Advance Settings" tab, then open "Edit Auto Fill Settings" dialog box.
3) To enable Auto-fill feature, set "Autofill" = "Enabled".
I hope this helps, please let us know if you need further assistance, we would be happy to assist you.