This example shows how to create a custom button on the viewer toolbar. First, load scripts:

@using Stimulsoft.Base
@using Stimulsoft.Report
@using Stimulsoft.Report.Blazor
@using Stimulsoft.Report.Web

Next, add report viewer:

<!--Report Viewer-->
<StiBlazorViewer ID="WebViewer1" Report="@report" OnAfterRenderEvent="@OnAfterRenderEvent" />

After that, create report object and load template:

@code
{
    //Report object to use in viewer
    private StiReport report;

    protected override void OnInitialized()
    {

        //Create report object and load template
        report = new StiReport();
        report.Load("Reports/TwoSimpleLists.mrt");
    }

...

Then, create a custom button:

<script>
	var createButton = function () {
		// 'jsWebViewer1' is a 'js' prefix plus the component ID
		var customButton = this.jsWebViewer1.SmallButton("customButton", "Custom Button", "emptyImage");
		customButton.image.src = "icon.png";
		customButton.action = function () {
			alert("Custom Button Event");
		}
		var toolbarTable = this.jsWebViewer1.controls.toolbar.firstChild.firstChild;
		var buttonsTable = toolbarTable.rows[0].firstChild.firstChild;
		var customButtonCell = buttonsTable.rows[0].insertCell(0);
		customButtonCell.appendChild(customButton);
	}
</script>

Finally, use InvokeVoidAsync to add a button:

...

	private void OnAfterRenderEvent()
	{
        JSRuntime.InvokeVoidAsync("createButton");
	}
}

In the screenshot below you can see the result of the sample code: