How to automatically update email field based on user field

I want to be able to automatically fill up the section Sales support email based on the sales user that is selected

So what ive done is stored the sales support email inside of the user that the one of my sales person uses.
I want to know whether this is possible in client script as i want it to appear on the creation page, if not is there any other way?

these are the APIs for the estimate module that has the support email and sales field.

Please guide me through as im not very sure with argument mapping.

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