The multi-Domains feature is available for the Business plan and higher plans users. It can be used to configure ss for multiple websites that need to use one server GTM container—for example, localisations for different languages (example.com, example.es, example.de, etc.). 

1. Go to your stape account and open your container. 

2. At the bottom of the page, click Power-Ups, scroll down to Multi-Domains and click Change plan and Upgrade to Business plan

3. If you are already subscribed to a Business plan, add all custom domains you need to use for sGTM. Once done, you would have to create an A record for all of the ss custom domains, similar to what you usually do for a single domain. 

For more detailed instructions on how to set up sGTM with multi-domain please check this article.