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Sorry but this is currently not possible. To better understand why that's the case, please have a look at the reply of one of our Developers on this thread.
That thread above was a canceled feature request due to the reasoning given on his reply. To sum it up, the Unique ID widget generates the ID "after form submission" so there's no way to show it while the user is filling out the form.
And even if we manage to come up with a way to generate it prior to form submission, issues and conflicts would still arise if multiple users open the form and never submitted it in the same order the IDs were generated (thus the impracticality of having such a feature).
As an alternative, if you really need something unique to be displayed on your form "while the user is filling it out", then try the Random Value Generator widget instead.
Here's a quick guide on How-to-Add-a-Widget-to-your-Form
In case you have a followup question, we'd appreciate it if you could give us more info and describe it in detail so we can better assist you.