tell application "Mail"
set myMessage to make new outgoing message with properties {visible:true, sender:"wifeaccount@mindspring.com "}
set message signature of myMessage to signature "signature name"
end tell
‘myMessage’ is just a variable that holds a reference to the created message that allows you to do things with it. It could be something different, ‘x’ perhaps, but using meaningful variable names will help make scripts readable.
you can even refer to the message without using a variable
tell application "Mail"
tell (make new outgoing message with properties {visible:true, sender:"wifeaccount@mindspring.com "})
set message signature to signature "signature name"
end tell
end tell