$w = Get-SPWebApplication -identity http://<WebApplication>
$w.GrantAccessToProcessIdentity("<Domain>\<Username>")

Note: Before running this command, the account should be registered as managed account

To register a managed account

1. On the SharePoint Central Administration website home page, in the left navigation, click Security.
2. On the Security page, under General Security, click Configure managed accounts.
3. On the Managed Accounts page, click Register Managed Account.
4. Type the user name and password of the domain account that you are registering.
5. Optionally, select the Enable automatic password change check box if you want SharePoint Server to manage password changes for this account.
6. Click OK.

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