@{
    var htmlTemplatesJson = JsonSerializer.Serialize(new[]
    {
        new { name = "Product Showcase", html = "<div style='border:2px solid ...'>...</div>" },
        new { name = "Team Profile", html = "<div style='display:flex;...'>...</div>" }
    });
}

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