Below you will find 2 methods on how to achieve your goal.
1. Manually, through AAD
2. Via PowerShell
What is Azure tenant ID?
Your Azure tenant ID is a globally unique identifier (GUID) that is different than your organization name or domain.
1. Manually, through AAD
Your tenant ID can be found in the Directory ID box on the Properties page in Azure Active Directory Admin center. 
2. Via Script
The Get-AzureADTenantDetail cmdlet gets the details of a tenant in Azure Active Directory (AD).
Run the script and login with appropriate permissions to the related tenant:
Using PowerShell,
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Connect-AzureAD
Get-AzureADTenantDetail 
