Say what you want, they have to click OK
Create Logon Messages for Users Signing in to Windows 11/10 [Tutorial]
Windows 7/8/10 lets you add a message that will be displayed on the logon screen when users log in. Use the Local Security Policy editor to set a message for users who log on to your computer.
The message is informational only and does not provide any real security. Depending on what you write, it may scare someone off, but the message will be removed once they click OK. Also, you can only edit security policy settings on Pro and higher versions of Windows. It does not work on Home or Starter editions.
Enter the following into the Search programs and files box on the Start menu and press Enter or click the link.