<TelerikButton OnClick="@AddNotification">Open a notification</TelerikButton>

<TelerikNotification VerticalPosition="@NotificationVerticalPosition.Top" @ref="@NotificationReference" Class="MyTelerikNotification">
    <Template>
        @{
            var item = context.Text;
            @((MarkupString)item.Replace("\n", "<br />"))
        }
    </Template>
</TelerikNotification>

@code {
    public TelerikNotification NotificationReference { get; set; }

    string message = "Firstline\n" + "Secondline\n" + "Thirdline\n";

    public void AddNotification()
    {
        NotificationReference.Show(new NotificationModel()
            {
                Text = message,
                ThemeColor = "primary"
            });
    }
}

<style>
    .MyTelerikNotification .k-notification-container .k-notification-wrap {
        width: 300px;
        height: 150px;
        font-size: 1.5em;
        text-align: center;
        align-items: center;
    }
</style>