Office 365 tenant have a tenant name and alphanumeric tenant ID, often when people ask for the tenant ID, they may just want the tenant name, but either way, here is how to find both:
Your tenant name is the name specified when you sign up in the format: company.onmicrosoft.com, you can usually find it in your company profile
Or domain Options:
To find it with PowerShell, first, install the Microsoft Online Services Sign-In Assistant for IT Professionals RTW from the Microsoft Download Center. Then install the Azure Active Directory Module for Windows PowerShell (64-bit version), and click Run to run the installer package.
Run PowerShell as Administrator, and run Connect-MsolService, you will be prompted for your credentials
Get-MsoAccountSku, the first element of the AccountSkuID is the tennant ID.
Alphanumeric Office 365 Tenant ID
You can find your tenant ID in the Azure AD portal. You’ll need to be an Azure AD administrator.
Find your Office 365 Tenant ID in the Azure AD portal
Under Azure Active Directory / Properties, see Directory ID
Find your Office 365 Tenant ID on the Office 365 Admin Center under the SharePoint Admin Portal
apps / app permissions
It’s the number behind the “@”
Find your Office 365 Tenant ID via PowerShell
Thanks, Office 365 Tech Expert in comments. See above to get your tenant name