First you must create a managed account for your win2k8r2sptest \ administrator. Here is the sequence of steps
$cred = get-credential 'win2k8r2sptest\administrator'
$adminMA = New-SPManagedAccount -Credential $cred
New-SPWebApplication -Name 'Test Webapp' -ApplicationPool "DefaultAppPool" -ApplicationPoolAccount $adminMA
SPS2010 SP1. , Foundation.