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. The subdomain is automatically verified by Microsoft Entra ID. To see that the subdomain you added is verified, refresh the domain list in the browser.
See: https://zcusa.951200.xyz/en-us/entra/identity/users/domains-manage#add-subdomains-of-a-custom-domain
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Abiola