Solved. The email address is created when you start a sandbox account.
Create a Microsoft Advertising Sandbox Account
Hello,
I'd like to create a Microsoft Advertising Sandbox account so my team can test API connections. The help documents state that an outlook-int.com email account is required to create the account.
"Sandbox supports MSAs created using an outlook-int.com email account only. You may not use an outlook.com email account. Also, you may not use an email account from another email service (for example, @contoso.com) even if the account is linked to an outlook.com or outlook-int.com email account."
https://learn.microsoft.com/en-us/advertising/guides/sandbox?view=bingads-13
How do I create this type of email account?
I had originally posted this question in the Microsoft Advertising community, but was asked to post my question here instead :).