Google Apps Script functions to pick a range and populate in text field in sidebar

pick_range.js

// Add the options to the custom menu
function onOpen() {
  SpreadsheetApp.getUi() // Or DocumentApp or FormApp.
      .createMenu('Custom Menu')
      .addItem('Show Settings', 'showSidebar')
      .addToUi();
}

// initiates the sidebar
function showSidebar() {
  var html = HtmlService.createHtmlOutputFromFile('settings')
      .setTitle('Settings Sidebar')
      .setWidth(300);
  SpreadsheetApp.getUi().showSidebar(html);
}

/** Function to pick the selected range from the Google Sheet
  * This returns the picked range, so that the client-side JS 
  * function (in HTML file) can populate it in the text field **/
function getSelectedRange(){
  var selected = SpreadsheetApp.getActiveSheet().getActiveRange(); // Gets the selected range
  var rangeString = selected.getA1Notation(); // converts it to the A1 type notation
  return rangeString;
}