Example code to pop a toast notification in either PowerShell desktop or core editions

Pop-ToastNotification.ps1

If ($PSVersionTable.PSVersion.Major -lt 6)
{
    $null = [Windows.UI.Notifications.ToastNotificationManager, Windows.UI.Notifications, ContentType = WindowsRuntime]
    $null = [Windows.Data.Xml.Dom.XmlDocument, Windows.Data.Xml.Dom.XmlDocument, ContentType = WindowsRuntime]
}
else
{
    # Check for the NuGet package provider and install it if necessary
    if ($null -eq (Get-PackageProvider -Name NuGet))
    {
        try
        {
            $null = Install-PackageProvider -Name NuGet -Force -Scope CurrentUser -ErrorAction Stop
        }
        catch
        {
            throw $_.Exception.Message
        }
    }
    # Check for the Microsoft.Windows.SDK.NET.Ref NuGet package and install it if necessary
    If ($null -eq (Get-Package -ProviderName NuGet -Name Microsoft.Windows.SDK.NET.Ref -AllVersions -ErrorAction SilentlyContinue))
    {
        try
        {
            $null = Install-Package -Name Microsoft.Windows.SDK.NET.Ref -ProviderName NuGet -Force -Scope CurrentUser -ErrorAction Stop
        }
        catch
        {
            throw $_.Exception.Message
        }
    }
    # Get the latest version of the WinRT.Runtime.dll and Microsoft.Windows.SDK.NET.dll files
    $WinRTRuntime = Get-ChildItem -Path "$env:LOCALAPPDATA\PackageManagement\NuGet\Packages\Microsoft.Windows.SDK.NET.Ref.*" -Filter "WinRT.Runtime.dll" -Recurse -ErrorAction SilentlyContinue |
        Sort-Object -Property VersionInfo.FileVersion -Desc | Select -ExpandProperty FullName | Select -First 1
    $WinSDKNet = Get-ChildItem -Path "$env:LOCALAPPDATA\PackageManagement\NuGet\Packages\Microsoft.Windows.SDK.NET.Ref.*" -Filter "Microsoft.Windows.SDK.NET.dll" -Recurse -ErrorAction SilentlyContinue |
        Sort-Object -Property VersionInfo.FileVersion -Desc | Select -ExpandProperty FullName | Select -First 1
    # Load the WinRT.Runtime.dll and Microsoft.Windows.SDK.NET.dll files
    Add-Type -Path $WinRTRuntime -ErrorAction Stop
    Add-Type -Path $WinSDKNet -ErrorAction Stop
}

# Notification parameters
$Title = "Hello matey!"
$AudioSource = "ms-winsoundevent:Notification.Default"
$SubtitleText = "I say, ol' chap, it's time for a brew. Polly put the kettle on, and all that. While you prepare a cuppa, we'll install some updates."

# Define the toast notification in XML format
[xml]$ToastTemplate = @"
<toast duration="long">
    <visual>
    <binding template="ToastGeneric">
        <text>Windows Updates - you know you love 'em</text> 
        <group>
            <subgroup>
                <text hint-style="title" hint-wrap="true" >$Title</text>
            </subgroup>
        </group>
        <group>          
            <subgroup>     
                <text hint-style="subtitle" hint-wrap="true" >$SubtitleText</text>
            </subgroup>
        </group>
    </binding>
    </visual>
    <audio src="$AudioSource"/>
</toast>
"@

# Load the notification into the required format
$ToastXml = New-Object -TypeName Windows.Data.Xml.Dom.XmlDocument
$ToastXml.LoadXml($ToastTemplate.OuterXml)
 
# Display
$App = "Windows.SystemToast.WindowsUpdate.MoNotification2"
[Windows.UI.Notifications.ToastNotificationManager]::CreateToastNotifier($App).Show($ToastXml)