Thank you for asking this question on the **Microsoft Q&A Platform. **
Unfortunately, you can add a subdomain.
If you want to add a subdomain name such as ‘europe.contoso.com’ to your organization, you should first add and verify the root domain, such as contoso.com.
Source: https://learn.microsoft.com/en-us/azure/active-directory/enterprise-users/domains-manage
Hope this helps,
Carlos Solís Salazar
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