Powershell for adding Menu item in Global Navigation in sharepoint 2013

if(-not(
Get-PSSnapin | Where { $_.Name -eq "Microsoft.SharePoint.PowerShell"})
) {
Add-PSSnapin Microsoft.SharePoint.PowerShell;
}

$Url = "http://sp2013:5559/"
$web = Get-SPWeb $Url;
 $CreateNavigationNodeMethod = [Microsoft.SharePoint.Publishing.Navigation.SPNavigationSiteMapNode]::CreateSPNavigationNode      

#Add Test4 link
         $headingNode = $CreateNavigationNodeMethod.Invoke("Test4","/SitePages/Test4.aspx", [Microsoft.SharePoint.Publishing.NodeTypes]::AuthoredLinkPlain, $web.Navigation.TopNavigationBar)
        $headingNode.Properties["Audience"]=";;;;" +"Test1"
     
        $headingNode.Update()

#Add Test3 link

        $headingNode = $CreateNavigationNodeMethod.Invoke("Test3","/SitePages/Test3.aspx", [Microsoft.SharePoint.Publishing.NodeTypes]::AuthoredLinkPlain, $web.Navigation.TopNavigationBar)
        $headingNode.Properties["Audience"]=";;;;" +"Test,Test1"      
        $headingNode.Update()



#Add Dropdown
        $Heading="Test Name Dropdown"
        $CreateNavigationNodeMethod = [Microsoft.SharePoint.Publishing.Navigation.SPNavigationSiteMapNode]::CreateSPNavigationNode      
           
        $headingNode = $CreateNavigationNodeMethod.Invoke($Heading, [System.String]::Empty, [Microsoft.SharePoint.Publishing.NodeTypes]::Heading, $web.Navigation.TopNavigationBar)
       
        $headingCollection = $headingNode.Children  
       
     

            $linkNode = $CreateNavigationNodeMethod.Invoke("Test Dropdown", "/_layouts/ApplicationPages/Test.aspx", [Microsoft.SharePoint.Publishing.NodeTypes]::AuthoredLinkPlain,$headingCollection)
            $linkNode.Properties["Audience"]=";;;;" +"Test2,Test3"
            $linkNode.Update()