@*The style tag is place here for convenience. In a production app the style rules should be part of your application's css files.*@

<style type="text/css">
    .myNotificationClass {
        z-index: 20000;
    }
</style>

<TelerikButton OnClick="@AddNotification">Add Notification</TelerikButton>

<TelerikNotification @ref="@NotificationComponent"
                     HorizontalPosition="@NotificationHorizontalPosition.Center"
                     VerticalPosition="@NotificationVerticalPosition.Top"
                     Class="myNotificationClass">
</TelerikNotification>

<TelerikButton Icon="window" OnClick=@(()=> WindowVisible = true)>Open Modal</TelerikButton>

<TelerikWindow Width="500px" Height="300px" Centered="true" @bind-Visible=@WindowVisible Modal="true">
    <WindowTitle>
        <strong>Modal Window</strong>
    </WindowTitle>
    <WindowActions>
        <WindowAction Name="Close" />
    </WindowActions>
    <WindowContent>
        <p>Click the close button to dismiss this modal dialog.</p>
    </WindowContent>
</TelerikWindow>


@code {
    public TelerikNotification NotificationComponent { get; set; }
    public NotificationVerticalPosition VerticalPosition { get; set; }
    public NotificationHorizontalPosition HorizontalPosition { get; set; }

    public bool WindowVisible { get; set; }

    public void AddNotification()
    {
        NotificationComponent.Show(new NotificationModel() { Text = "Test Notification", ThemeColor = "success" });
    }
}

I've chosen a value of 20000 for the notification's z-index because the Window has its own z-index set to 10001 (see attached screenshot).