You can find Azure Active Directory domain in three easy steps.
Step 1: Login into Azure Management Portal
First, login into Azure Management Portal at https://portal.azure.com.
Step 2: Find Active Directory Icon in the left menu bar
Click on "Azure Active Directory" on the left hand menu bar.
Step 3: Find Domain
Find Azure AD name as shown in the image below (e.g. cerebratademo.onmicrosoft.com).
You can also associate a custom domain (e.g. mydomainname.com) with an Azure Active Directory. If a custom domain is associated with an Azure Active Directory,
domains tab will show that along with default domain. You can use either one of them.