Alert new customers on the sign-up page that the initial ID "***@xxxxx.onmicrosoft.com" is used for Office 365 Admin's account
Customers tend to get confused with the initial "***@xxxxx.onmicrosoft.com" account when they access to Office 365 for the first time right after signed-up tenants.
This is because there is no notification or alert on the "Create your user ID" page to let customers know that it's going to be an initial Admin ID.
Also, as it's too long for very new customers, they easily forget their Admin ID or memorize in a wrong spellings.
It makes the customers sign-up again and request account validation.