SQL Server Connection String for Integrated Security (Windows Authentication)
connectionString="data source=SERVER\INSTANCE;Initial Catalog=MyDB;Integrated Security=SSPI;
If using an App Pool account, use the following example formats:
where xxxx is the name of the App Pool in IIS.
There is no need to create the App Pool account as a windows user, but you will need to create it as a Login on the SQL Server, specifying Windows Authentication as the login type.