JotForm is a free online form builder which helps you create online forms without writing a single line of code. No sign-up required.
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JotForm Support Manager
After enabling the "Input Mask" button at the tool bar
You can choose the format on Mask Value section. You can even include some predefined values such as the area numbers. You can even choose to fill all the values at once if this is a field for a fixed number
Here is a demo