CK editor5 React - Add custom button

I have implemented react CK editor using the tutorial https://ckeditor.com/docs/ckeditor5/latest/builds/guides/integration/frameworks/react.html

The code is as follows.

import React, {Component} from 'react';
import CKEditor from '@ckeditor/ckeditor5-react';
import ClassicEditor from '@ckeditor/ckeditor5-build-classic';

class App extends Component {
    render() {
        return ( 
            <div className = "App">                
            <h2> Using CKEditor 5 build in React < /h2> 
            <
            CKEditor editor = {
                ClassicEditor
            }
            data = "<p>Hello from CKEditor 5!</p>"
            onInit = {
                editor => {
                    // You can store the "editor" and use when it's needed.
                    console.log('Editor is ready to use!', editor);
                }
            }
            onChange = {
                (event, editor) => {
                    const data = editor.getData();
                    console.log({
                        event,
                        editor,
                        data
                    });
                }
            }
            />
            </div>
        );
    }
}

export default App;

Now i need to add a custom button to the toolbar which when clicked a function needs to be called.

I have seen posts on adding new plugins but i am not sure on how to implement this in the react way.

Any help would be appreciated.