example of writing extension to change behavior of share file

index.ts

// src/index.ts 
import {
  JupyterFrontEnd, JupyterFrontEndPlugin
} from '@jupyterlab/application';

import { Clipboard } from '@jupyterlab/apputils';

import { URLExt } from '@jupyterlab/coreutils';

import { IFileBrowserFactory } from '@jupyterlab/filebrowser';

import { toArray } from '@phosphor/algorithm';

function activateShareFileMYAP(
    app: JupyterFrontEnd,
    factory: IFileBrowserFactory
): void {
    const { commands } = app;
    const { tracker } = factory;
    //commands.addCommand('my.jupyterlab', { // wrong!!!
    commands.addCommand('filebrowser:share-main', {
        execute: () => {
            const widget = tracker.currentWidget;
            if (!widget) {
                return;
            }
            const path = encodeURI(widget.selectedItems().next().path);
            Clipboard.copyToSystem(URLExt.join('myap://Notebook', path));

        },
        isVisible: () =>
            tracker.currentWidget &&
            toArray(tracker.currentWidget.selectedItems()).length === 1,
        iconClass: 'jp-MaterialIcon jpLinkIcon',
        label: 'Copy Shareable Link'
    });
}

/**
 * Initialization data for the my.jupyterlab extension.
 */

const extension: JupyterFrontEndPlugin<void> = {
  activate: activateShareFileMYAP,
  id: 'my.jupyterlab:factory',
  requires: [IFileBrowserFactory],
  autoStart: true

};

export default extension;