Your example code attempts to create a new Redactor instance, which (as Brad says) won't work inside the control panel (where Craft has created the Redactor instances already).

You can, however, use the Redactor config files to add an element with the classname .note. To do that, add the following to the file config/redactor/Standard.json (or if you configured your RichText field to use a different configuration, the JSON file corresponding to that configuration):

"formattingAdd": {
  "note": {
    "title": "Note",
    "args": ["p", "class", "note"]
  }
}

Note that the first parameter in the args property is the tag which the .note element will use – according to the Redactor docs, only p, pre, blockquote, div and heading tags can be used with formattingAdd.

Also note that unless you add some custom CSS, the .note will – of course – not be styled any differently than a normal paragraph.

Unfortunately, there's no way to add custom CSS and JavaScript to the Control Panel without a custom plugin – either by writing your own or by installing the CP CSS and CP JS plugins.

Your example code attempts to create a new Redactor instance, which (as Brad says) won't work inside the control panel (where Craft has created the Redactor instances already).

You can, however, use the Redactor config files to add an element with the classname .note. To do that, add the following to the file config/redactor/Standard.json (or if you configured your RichText field to use a different configuration, the JSON file corresponding to that configuration):

"formattingAdd": {
  "note": {
    "title": "Note",
    "args": ["p", "class", "note"]
  }
}

Note that the first parameter in the args property is the tag which the .note element will use – according to the Redactor docs, only p, pre, blockquote, div and heading tags can be used with formattingAdd.

Also note that unless you add some custom CSS, the .note will – of course – not be styled any differently than a normal paragraph.

Unfortunately, there's no way to add custom CSS and JavaScript to the Control Panel without a custom plugin – either by writing your own or by installing the CP CSS and CP JS plugins.