There are 2 ways to add admin user in to an instance.
1. System Administrator User Import from Acive Directory.
2. Using admin user provisioning tool.
Using admin user provisioning tool.
1. Run the AdminUserProvisioning shortcut placed in desktop of AOS server.
2. Enter the email address of the account has AAD ( Azure Active directory) and Submit.
Note: Shortcut is not available in desktop, please check below URL