Set up the report viewer in WebAssembly, load a template, and connect the component to the application.

Viewer setup. Configure the viewer, load the report, assign it to the component, and process the requests or events raised by the UI.
@inject IJSRuntime JSRuntime

<StiBlazorViewer ID="WebViewer1" Report="@Report" OnViewerAfterRender="@OnViewerAfterRender" />

@code {
    private void OnViewerAfterRender()
    {
        JSRuntime.InvokeVoidAsync("createButton");
    }
}
The OnViewerAfterRender callback fires when the viewer is ready; from it you invoke a JavaScript function (createButton) that inserts a custom button into the toolbar and wires it to your logic through the viewer's client API.
На скриншоте ниже Вы можете увидеть результат выполнения данного кода:

Creating a Custom Button on the Viewer Toolbar