Adding a message of the day for your Terraria server can be completed as follows:

Navigate to your Clanforge page and click on the 'Profiles' tab on the left hand side. Now, as you would like to edit some settings, you will need to click the Edit button at the bottom and then click on the Config tab at the top. Once within the Config tab you can now navigate to the serverconfig.txt. Within this config file you can change the line, "motd=" to whatever you would like the message to be. A pictorial example is shown below.