The Office 365 Tenant ID is pretty good hidden. Therefore here are two was finding out what's your Tenant ID ist.
Open PowerShell and login into Office 365
Connect-MsolService -Credential $credential
$TenantID = (Invoke-WebRequest https://login.windows.net/YourOFFICE365TenantName.onmicrosoft.com/.well-known/openid-configuration|ConvertFrom-Json).token_endpoint.Split(‘/’)
Finding the Tenant ID in SharePoint
Author: Thomas Poett MVP, Business Unit Lead Microsoft Unified Communication