@{
    var htmlTemplatesJson = JsonSerializer.Serialize(new[]
    {
        new { name = "Email - Welcome", html = "<h2>Welcome...</h2>..." },
        new { name = "Report - Monthly", html = "<h1>Monthly Report...</h1>..." },
        new { name = "Blog - Post", html = "<h1>[Blog Post Title]</h1>..." }
    });
}

<richtextbox
    name="templateCategoriesEditor"
    toolbar="full"
    height="400px"
    html-templates-json="@htmlTemplatesJson" />