say the field with the email address is named TextField1
you can add a plain button and add the following script to its click event
var emailAddress = TextField1.rawValue;
{
event.target.submitForm({cURL:"mailto:"+emailAddress+"?subject=TEST&body=TEST.",cSubmitAs:"XML",cCharset:"utf-8"});
}
so.. the button becomes an email submit button that fills the To: field with the email address previously provided by the user
if it doesn't work, you can punch a wall, thats what i do