<TelerikWindow Modal="true"
               Visible="@isModalVisible"
               VisibleChanged="@VisibleChangedHandler"
               CloseOnOverlayClick="true">
    <WindowTitle>
        Window Title
    </WindowTitle>
    <WindowContent>
        I am modal, so the page content behind me is not accessible to the user.
    </WindowContent>
    <WindowActions>
        <WindowAction Name="Close" OnClick="@CloseWindow" />
    </WindowActions>
</TelerikWindow>

<TelerikButton OnClick="@( _ => isModalVisible = true )">Open the window</TelerikButton>

@code {
    private bool isModalVisible { get; set; } = true;

    private void VisibleChangedHandler(bool currVisible)
    {
        if (currVisible)
        {
            isModalVisible = currVisible;
        }
    }

    private void CloseWindow()
    {
        isModalVisible = false;
    }
}