Powershell Set IIS apppool custom identity credential

import-module webadministration

function Set-WebsiteAppPoolCustomIdentity([String] $appPoolName, [String] $userId, [String] $pwd)
{ 
    Write-Host $appPoolName;
    $webAppPool = get-item IIS:\AppPools\$appPoolName     
    $webAppPool.processModel.userName = $userId;
    $webAppPool.processModel.password = $pwd;
    $webAppPool.processModel.identityType = 3;
    $webAppPool | Set-Item
    $webAppPool.Stop();
    $webAppPool.Start();
    Write-Host "IIS Recycled";
    $webAppPool = get-item iis:\apppools\$appPoolName;
    write-host "New Pool User: " $webAppPool.processModel.userName;
    write-host "New Pool PWd: " $webAppPool.processModel.password;
}

Set-WebsiteAppPoolCustomIdentity  -appPoolName "abc" -userId "hello" -pwd "world"