In your Mac login window, you can add a welcome message right below the Apple and System Status. You only need to perform this following command line:
sudo defaults write /Library/Preferences/com.apple.loginwindow LoginwindowText "Hi, I am Mac"
Characters inside double quote will be displayed as your welcome message. I know you will try this and I also know that you will want to delete it as soon as you've finished trying. So, here is the command line to delete your Mac welcome message:
sudo defaults write /Library/Preferences/com.apple.loginwindow LoginwindowText ""
Someday, you can consider putting your username and password as the welcome message. No one will know, except me, you and other readers of UsingMac.com.
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