Instructions
Open the directory into which you installed your "Counter-Strike: Source" server. Typically, that directory is named "cstrike." If your server is on a remote computer, you can either use Windows' remote connection tool or an FTP client to connect to it. If your "Counter-Strike: Source" server is on your own computer, open that directory in Windows Explorer.
Double-click on the "motd.txt" file. The computer will now automatically launch Notepad and open the file.
Replace the default MOTD by any message you wish to display to players connecting to your server.
Save the file by clicking on "File" and selecting "Save."