How to add annotation to pdf file using Syncfusion EJ2 PDF Viewer for ASP.NET Core control from code?

Question

I am trying to figure out how to add custom stamp to a pfd document using Syncfusion PdfViewer for ASP.NET Core. From documentation: https://ej2.syncfusion.com/aspnetcore/PdfViewer/Annotations#/fluent you can use such code:

pdfViewer.annotation.addAnnotation("Area", {
            offset: { x: 200, y: 500 },
            pageNumber: 3,
            vertexPoints: [{ x: 200, y: 500 }, { x: 288, y: 499 }, { x: 289, y: 553 }, { x: 200, y: 500 }]
        });

However when I use it, I get this: "Uncaught TypeError: pdfViewer.annotation.addAnnotation is not a function". Am I doing something wrong?

Here is some more context:

<div style="width:100%;height:1200px">
//Viewer
       <ejs-pdfviewer id="pdfviewer" serviceUrl="/api/PdfViewer" documentPath="HTTP Succinctly.pdf" enableToolbar="true"
    documentLoad="documentLoaded" ></ejs-pdfviewer>
</div>

<script type="text/javascript">
function documentLoaded(arg) {
    var pdfViewer = document.getElementById('pdfviewer').ej2_instances[0];
    if (arg.documentName === "HTTP Succinctly.pdf") {
        pdfViewer.annotation.addAnnotation("Area", {
            offset: { x: 200, y: 500 },
            pageNumber: 3,
            vertexPoints: [{ x: 200, y: 500 }, { x: 288, y: 499 }, { x: 289, y: 553 }, { x: 200, y: 500 }]
        });
    }
};
</script>