Hello and welcome to the forums,
It is correct that the variable that controls the message displayed is STARTING_MESSAGE.
The right place to modify that value is to add the following line to your data/config.php:
define('STARTING_MESSAGE','your message here');
Please note that modifying this value in /include/config.php is highly discouraged, since this way the change would be reverted on the next update. Having it applied in /data/config.php will work and makes it easy to upgrade and keep this modification.
Kind regards,
Arek