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Yes you can pre populate also the iframe published form.
It use to work before as mentioned here
But for some reason, it is not working today. There seems to be a glitch on this feature.
I will send a ticket report to our higher team support regarding this bug. Hopefully it can be restored the soonest time possible.
We will let you know here once we have update on the status.
we have fixed it. You need to republish your form to iframe.
Thanks for using JotForm.