This is an optional step. Each web application uses a specific application pool, which uses a specific Windows account.
The Windows account used by the application pool can be changed as follows:
Click the Application Pools node and right-click the application pool whose name matches the virtual directory name of the Portal (for example, Portal).
Select Advanced Settings and open the Identity setting.
Select Custom Account and Set the credentials that you want to use.
Figure 1. IIS Manager - Change SQL Server account access