This article explains how to find your Office 365 tenant name.
From the Office 365 Admin Center
If you have global admin rights, Under Setup click on Domain
Locate the domain that ends with .onmicrosoft.com
The office 365 tenant name is the left part of .onmicrosoft.com
OneDrive for Business Service
Login to Office 365.
Click on the link to open your OneDrive.
The url will look like https://tenantname-my.sharepoint.com/personal/<your user name>/_layouts/15/onedrive.aspx
Your tenant name is the left part of -my
Click on the SharePoint Online
This opens a url like https://tenantname.sharepoint.com/_layouts/15/sharepoint.aspx
Tenant name is the left part .sharepoint.com