Please respond to your original post so that all concerned can read what has gone before. I was puzzled by this message until I remembered that you had posted this same question earlier.
You really need to read my last post on the original thread again.
You cannot do exactly what you want to do here. FileMaker will not allow you to.
I posted a work around that achieves the same result. Put your email information into fields such as a text field for the message body. You can review and edit these fields as much as you need to. Don't use send mail until you are ready to actually send the email. When you do use Send Mail, the Send Mail dialog or script step can use the text you've typed into these fields to create the email to be sent.