How to generate a welcome message while opening the AX 2012
Set a startup message
Open the Microsoft Dynamics AX 2012 Configuration utility. In Windows, click Start > Control Panel > Administrative Tools > Microsoft Dynamics AX 2012 Configuration.
Click Manage > Create Configuration.
Enter a name for the configuration and then click Active configuration.
On the General tab, in the Startup message box, type the message you want to be displayed, and then click OK.When the client is started, the text is displayed as a message that includes an OK button.
Open AX Client Configuration
Open As Administrator
and then you can input your massage as needed in right highlighted box as show in the screen shot: