Impossible to modify tinymce settings for props on local block properties

I have a page with a "local" block. (Block used as a property on a page). This block has a XhtmlString property. But I can not change the configured TinyMCE editor for this property.

I tried used the documented approach (https://world.episerver.com/documentation/developer-guides/CMS/add-ons/customizing-the-tinymce-editor-v2/default-settings/), but this failed.

 public void ConfigureContainer(ServiceConfigurationContext context)
        {
            context.Services.Configure<TinyMceConfiguration>(config =>
            {
                var simpleConfiguration = config.Default().Clone()
                  .Toolbar("someFeature someFeature2");
                config.For<StartPage>(a => a.ChatSettingsBlock.WelcomeMessage, simpleConfiguration);
            });
        }