Take a look to these threads https://forum.studio-397.com/index.php?threads/how-to-hosting-with-steamcmd-and-paid-content.57209/ https://forum.studio-397.com/index.php?threads/how-to-creating-a-raceevent-with-paid-content.57207/
Have you gone through the process for running DLC on your server ? It should say keys loaded in the menu bar of modmanager if you have