@inject DialogService DialogService

<RadzenHtmlEditor style="height: 200px;" Execute=@OnExecute>
    <RadzenHtmlEditorCustomTool CommandName="InsertImageFromList" Icon="attach_file" />
</RadzenHtmlEditor>

@code {
    async Task OnExecute(HtmlEditorExecuteEventArgs args)
    {
        if (args.CommandName == "InsertImageFromList")
        {
            await InsertImageFromList(args.Editor);
        }
    }

    async Task InsertImageFromList(RadzenHtmlEditor editor)
    {
        await editor.SaveSelectionAsync();

        var result = await DialogService.OpenAsync<HtmlEditorCustomDialog>("Select image file"); 

        await editor.RestoreSelectionAsync();

        if (result != null)
        {
            await editor.ExecuteCommandAsync(HtmlEditorCommands.InsertHtml, $"<img alt=\"Selected image file preview\" style=\"max-width: 100%\" src=\"{result}\">");
        }
    }
}