Using the Send Mail script step will allow you to specify a field that contains the email address you want to send to.
You can do something along the lines of:
If [Get ( LastMessageChoice ) = 1] //1 being the submit button
Send Mail [parameters]
Exit Script //or whatever else you would like script to do if submit is not clicked
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However, prior to the send mail script step, you may need to add a commit record step to be sure the previous field is stored.