Explanation

Step 1: Set up the Active Directory Admin for AzSQL1.
Step 2: Connect to DB1 by using the server administrator.
Sign into your managed instance with an Azure AD login granted with the sysadmin role.
Step 3: Create a user by using the FROM EXTERNAL PROVIDER clause.
FROM EXTERNAL PROVIDER is available for creating server-level Azure AD logins in SQL Database managed instance. Azure AD logins allow database-level Azure AD principals to be mapped to server-level Azure AD logins. To create an Azure AD user from an Azure AD login use the following syntax:
CREATE USER [AAD_principal] FROM LOGIN [Azure AD login]
Reference:
https://docs.microsoft.com/en-us/sql/t-sql/statements/create-user-transact-sql