function onOpen() { var ui = SpreadsheetApp.getUi(); ui.createMenu('Custom Menu') .addItem('Open Custom Dialog', 'openCustomDialog') .addToUi(); } function openCustomDialog() { var html = HtmlService.createHtmlOutputFromFile('Page') .setWidth(300) .setHeight(200); SpreadsheetApp.getUi().showModalDialog(html, 'Custom Dialog'); } function processSelection(selection) { var sheet = SpreadsheetApp.getActiveSpreadsheet().getActiveSheet(); sheet.getRange('A1').setValue(selection); }