Most hosts will require the From Address to be a valid email for the hosting domain. there's not really a way around that.
In Universal Email there is a setting for the Reply To address (click the From button in the UE wizard to access advanced from settings and the reply to address).
you can set the reply to address to use the dynamic email from the form. that will have the effect that the email comes from the static address for your domain, but when replying to the email, the reply will use the dynamic address entered in the form.