<TelerikButton OnClick="@DoLongWork">Do long work</TelerikButton>

<TelerikWindow @bind-Visible="@LoadingSignVisible" Modal="true">
    <WindowTitle>
        <strong>Please Wait</strong>
    </WindowTitle>
    <WindowContent>
        @*Add animated gif and styling to taste*@
        Please wait...we are processing your request.
    </WindowContent>
</TelerikWindow>

@code {
    bool LoadingSignVisible { get; set; }

    //this method must be async so the UI can be updated while waiting for the remote service
    async Task DoLongWork()
    {
        LoadingSignVisible = true;

        await Task.Delay(3000);//simulate long running operation

        LoadingSignVisible = false;
    }
}