This article explains how to find your SharePoint URL Prefix.
In the O365 App Launcher, click on the SharePoint Online
This opens a url like https://tenantname.sharepoint.com/_layouts/15/sharepoint.aspx
Your tenant name is the text to the left of .sharepoint.com
OneDrive for Business Service
Login to Office 365.
In the App Launcher, 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