this action will allow you to message anyone on the network even across broadcast domains.
you will need to place the batch file in your custom action folder. you will also need psexec in your CA folder as well
CA CODE
cmd.exe /K "{actionpath}messager.bat {actionpath} {ipaddress}
past this in messager.bat
@echo off
WMIC /Node:%2 ComputerSystem Get UserName
:input
set INPUT=
set INPUT2=
echo.
echo.
echo.
set /P INPUT=Type Username to Message from list: %INPUT%
set /P INPUT2=Type Message here: %INPUT2%
%1psexec.exe \\%2 msg %INPUT% %INPUT2%
exit
this will list the current users that are logged in and then it will message a user of your choosing