1. Log in to your GTM server account and click Add custom domain.
2. Log in to your domain name service and create a new DNS record for the subdomain you want to use.
3. Once your GTM subdomain is added and verified, you should see the status Running.
4. Go to your Google Tag Manager Server container -> Admin -> Container Settings -> Change Tagging server URL to your subdomain.
And that's it! You can find out more in our article on how to add a custom domain to the Google Tag Manager Server container and if you need help, just contact us.