Ensure that names like ‘Admin’ are not used for Azure SQL Serv…

Creating administrative users in SQL can be done at multiple stages, however the initial administrator username can only be set at the time the instance is created; changing the username will require a new instance to be created. Before starting, ensure that the user you wish to use is already configured in Azure AD. To create a new instance, follow the steps below.

In Azure Console –

Open the Azure Portal and go SQL Servers.
Select the Create button.
On the Basics tab, ensure that Authentication Method is set to Azure AD.
In the Set Azure AD admin field, select Set admin.
Select the user to use from the list.

In Terraform –
For current Azure Provider versions:

In the azurerm_mssql_server resource, create an azuread_administrator block.
Set login_username to a value that doesn’t include well known username values.

For Azure Provider versions prior to 2.99.x:

In the azurerm_sql_active_directory_administrator resource, set login to a value that doesn’t include well known username values.
Examples of well known values: azure_superuser, azure_pg_admin, admin, administrator, root, guest, public

References:
learn.microsoft.com/en-us/azure/azure-sql/database/authentication-aad-service-principal-tutorial?view=azuresql
registry.terraform.io/providers/hashicorp/azurerm/latest/docs/resources/mssql_server
registry.terraform.io/providers/hashicorp/azurerm/2.99.0/docs/resources/sql_active_directory_administrator