How to add custom domain names to a GCC tenant.

In the GCC cloud, custom domains need to be added using Azure AD PowerShell for Office 365 GCC High tenants.
1. First, ensure you have installed the Azure AD PowerShell module.In the Administrator: Windows PowerShell command window, run this command:
Install-Module -Name AzureAD

2. Use the New-AzureADDomain cmdlet to add the domain to Azure AD.
New-AzureADDomain -name contoso.com

3. Use the Get-AzureADDomainVerificationDnsRecord to retrieve the details necessary to be entered onto the domain’s DNS zone (this is to prove ownership of the domain).
Get-AzureADDomainVerificationDnsRecord -Name contoso.com | fl

The output will look like the following image:

4. Add the TXT record for verification with the customer’s DNS hosting provided.

5. After the verification record has been added to the public DNS servers and successfully propagated across the internet, use the following command to validate ownership
Confirm-AzureADDomain -Name contoso.com

The command will not return successfully until DNS replication is completed. This could take up to 72 hours due to replication. It has no dependency on Microsoft network.