How to create new message in iChat without sending, using Automator to grab selected text
You have to do it with UI scripting : 

tell application "System Events" to tell process "Messages"
    set input to "http://aol.com" as text
    click button 1 of group 1 of splitter group 1 of window 1
    delay 1
    keystroke "Buddy@provider.com" -- type the reciever
    keystroke return -- validate the previous input
    keystroke tab -- move to message input
    keystroke input -- type the message
end tell