Create shared mailbox with RES Automation Manager and Powershell

I created a script for our servicedesk that can create a shared mailbox in combination with RES Automation Manager.
Normally our servicedesk support staff can't create a shared mailbox because of the limited rights they have in our environment. Shared mailboxes can only be created with powershell and Organization Management rights.

The script runs as a "Module" in RES AM.
There are 2 variables that need to be entered:

1. The shared mailbox Alias
2. The shared mailbox description
   

The server on which the Module has to be run (CAS-HUB)

The 2 variables:



When the job has run successfully the following has happened:

 The sharedmailbox "Testmailbox" has been created in the OU domain.lan\Central\Accounts\Shared Mailboxes, and the login has been disabled. (by default for a shared mailbox)

A group for the shared mailbox "Testmailbox" has been created in the OU domain.lan\Central\Accounts\Groups\Shared Mailboxes with the name; g.f.mail.Alias (g.f.mail.testmailbox)

The new group is added to the shared mailbox with "Full Access" permissions and "Send as" permissions.
The only thing that has to be done by hand, is add the opropriate users to the new "Testmailbox".

The script:

param ($Alias,$DisplayName)
add-pssnapin Microsoft.Exchange.Management.PowerShell.E2010
import-module activedirectory
# Setup variables
$DomainController="sr-XXXXX.domain.lan"
$OU='domain.lan/Central/Accounts/Shared Mailboxes'
$UPNdomain = "@domain.com"
if ($Alias)
{
    if ($Alias.Contains('@')) { $Alias = $Alias.Substring(0,$Alias.IndexOf('@')) }
    $AliasMailbox = Get-Mailbox $Alias -ErrorAction SilentlyContinue
    $AliasMailUser = Get-MailUser $Alias -ErrorAction SilentlyContinue
    if ($AliasMailbox -or $AliasMailUser)
    {
        Write-Output "The Alias specified already exists."
        $Alias = $null
    }
}
while (!$Alias)
{
    $Alias = Read-Host -Prompt "Alias"
    if ($Alias)
    {
        if ($Alias.Contains('@')) { $Alias = $Alias.Substring(0,$Alias.IndexOf('@')) }
        $AliasMailbox = Get-Mailbox $Alias -ErrorAction SilentlyContinue
        $AliasMailUser = Get-MailUser $Alias -ErrorAction SilentlyContinue
        if ($AliasMailbox -or $AliasMailUser)
        {
            Write-Output "The Alias specified already exists."
            $Alias = $null
        }
    }
}
if ($DisplayName)
{
    $DisplayNameMailbox = Get-Mailbox $DisplayName -ErrorAction SilentlyContinue
    $DisplayNameMailUser = Get-MailUser $DisplayName -ErrorAction SilentlyContinue
    if ($DisplayNameMailbox -or $DisplayNameMailUser)
    {
        Write-Output "The Display Name specified already exists."
        $DisplayName = $null
    }
}
while (!$DisplayName)
{
    $DisplayName = Read-Host -Prompt "Display Name"
    if ($DisplayName)
    {
        $DisplayNameMailbox = Get-Mailbox $DisplayName -ErrorAction SilentlyContinue
        $DisplayNameMailUser = Get-MailUser $DisplayName -ErrorAction SilentlyContinue
        if ($DisplayNameMailbox -or $DisplayNameMailUser)
        {
            Write-Output "The Display Name specified already exists."
            $DisplayName = $null
        }
    }
}
#Alias to lower
$Alias=$Alias.ToLower()
# Set UPN
$UPN=$Alias + $UPNDomain
Write-Output "Creating Shared Mailbox"
New-Mailbox -Name:$Alias -Alias:$Alias -DisplayName:$DisplayName -OrganizationalUnit:$OU -UserPrincipalName:$UPN -SamAccountName:$Alias -Database:$MailboxDatabase –Shared –DomainController $DomainController
Sleep 10
Write-Output "Creating Security Group"
New-ADGroup -Name "g.f.mail.$Alias" -SamAccountName g.f.mail.$Alias -GroupCategory Security -GroupScope Global -DisplayName "g.f.mail.$Alias" -Path "OU=Shared Mailboxes,OU=Groups,OU=Accounts,OU=Central,DC=domain,DC=lan" -Description "Group for shared mailbox $Alias" 
Write-Output "Setting Mailbox Permissions"
sleep 50
Add-MailboxPermission –Identity: $Alias –AccessRights:FullAccess –user:"g.f.mail.$Alias"
Add-ADPermission –Identity: $Alias –user:"g.f.mail.$Alias" –ExtendedRights: 'Send-as'