Passing a Text Field to a Hidden Field to the Submission?

Passing a Text Field to a Hidden Field to the Submission?

I would like to know how to pass info from one text field to a hidden
field and then from the hidden field will change where that form is
actually delivered.
So for instance:
TextField1 = User enters a number
TextField2 = User enters their Name
TextField3 = User enters their Address
As entered:
TextField1 = 135820
TextField2 = John Doe
TextField3 = johndoe@mail.com
Upon clicking the Submit Button I'd like to have the form information sent
to an email address that consists of TextField1 & the domain name, where
the domain name never changes but the TextField1 does: 135820@mysite.com,
instead of a predefined email address, how normal forms are created.