Based on numerous posts about this, I have been dabbling with adding a custom button to the RTE. The button simply creates a

• Item

The direction I have taken was to add a new "plugin" to www/umbraco/lib/tinymce/plugins/customlist/plugin.min.js:

tinymce.PluginManager.add('customlists', (editor, url) => {
    editor.ui.registry.addButton('customlist', {
        text: 'Custom List',
        onAction: () => {
            /* Open window */
            editor.insertContent('&nbsp;<ul class="custom-list"><li>item</li></ul>&nbsp;');
        }
});
/* Return the metadata for the help plugin */
return {
    getMetadata: () => ({
        name: 'customlist',
        url: 'http://exampleplugindocsurl.com'
    })
};
});

This is based on the syntax from TinyMCE for the v6 of their editor. In my appsettings.json, I have added the below under the Umbraco.CMS namespace:

"RichTextEditor": {
  "Plugins": [
    "anchor",
    "charmap",
    "table",
    "customlist"
  ],
  "ValidElements": "+a[id|style|rel|data-id|data-udi|rev|charset|customlist|hreflang|dir|lang|tabindex|accesskey|type|name|href|target|title|class|onfocus|onblur|onclick|ondblclick|onmousedown|onmouseup|onmouseover|onmousemove|onmouseout|onkeypress|onkeydown|onkeyup],-strong/-b[class|style],-em/-i[class|style],-strike[class|style],-u[class|style],#p[id|style|dir|class|align]]",
  "InvalidElements": "font",
  "Commands": [
    {
      "Alias": "ace",
      "Name": "Source code editor",
      "Mode": "Insert"
    },
    {
      "Alias": "removeformat",
      "Name": "Remove format",
      "Mode": "Selection"
    },
    {
      "Alias": "paste",
      "Name": "Paste",
      "Mode": "All"
    },
    {
      "Alias": "customlist",
      "Name": "customlist",
      "Mode": "All"
    }
  ],
  "CustomConfig": {
    "Toolbar": "customlist"
  }
},

In Umbraco, I have edited the document type > richtexteditor and I can see the "customlist" is available and selected. I have also added an alert("test") within the plugin.min.js in the plugin source code and can confirm the alert does fire (even within the tinymce.PluginManager.add('customlists', (editor, url) => {} call, so the plugin itself is definately loaded.

Problem however is that the button (or text rather) does not show within the toolbar. I suspect that I need to specify in which toolbar, but the base configuration in the appsettings.json does not have seem to look out for the toolbar property. I have added it to the CustomConfig namespace, but no difference.

Or, it could be my code, but I tested it in codepen and the Toolbar item does show, so I suspect it's down to the initialization part. Any ideas?