Before you can start with Azure, you need an Azure Subscription. Each and every Azure Subscription must be associated with an Azure AD Tenant. This is sometimes referred to as a directory.
For those who have Office 365, one would have been created for you automatically as part of your deployment. If you don’t have/use Office 365, then a tenant can be created as part of the process required to sign up for an Azure Subscription.
All Azure AD tenants have a naming format as follows: %uniquestring%.onmicrosoft.com e.g. contoso.onmicrosoft.com
However, for several services you may need to use the unique ID rather than the name. Here is how to find it.
1. Log into Azure AD via the Azure Portal, here is a direct link to the Azure AD blade
2. Once on the Azure AD blade, scroll down and click on the Properties option
3. This opens a new blade, which gives you all the specific details relevant to your tenant. Right in the middle is your tenant ID, there is a copy button to the right.