<RadzenHtmlEditor style="height: 200px;">
    <RadzenHtmlEditorCustomTool>
        <Template Context="editor">
            <RadzenDatePicker Change=@(args => OnDateChange(args, editor)) TValue="DateTime" InputAttributes="@(new Dictionary<string,object>(){ { "aria-label", "select date" }})" />
        </Template>
    </RadzenHtmlEditorCustomTool>
</RadzenHtmlEditor>

@code {
    async Task OnDateChange(DateTime? date, RadzenHtmlEditor editor)
    {
        if (date != null)
        {
            await InsertDate(editor, date.Value);
        }
    }

    async Task InsertDate(RadzenHtmlEditor editor, DateTime date)
    {
        await editor.ExecuteCommandAsync(HtmlEditorCommands.InsertHtml, $"<strong>{date.ToLongDateString()}</strong>");
    }
}