Powershell to create Virtual directory with username

 Hello, I'm new to working with powershell and IIS.  I'm trying to figure out how to create a virtual directory via powershell and specifies the "connect as" username and password.

I can create the Virtual directory with:

 New-Item 'IIS:\Sites\www.test-site.com\robert-test\TestVirtual' -type VirtualDirectory -physicalPath \\FileServer\networkShare

 But how do I add the  "connect as" user information?

Sorry for the probably simple question.  Thanks!