At the top of your main.js, make sure the following variables are defined:

const ipc = ipcMain;
const { app, BrowserWindow, ipcMain, webContents } = require('electron');

Inside your button onclick function (in the renderer), use the following:

ipc.send('SomeEvent');

Also make sure the following are defined in your renderer

const { ipcRenderer } = require('electron');
const ipc = ipcRenderer;

Then, use the following to open in the external browser (it will open in the user's default)

ipc.on('SomeEvent', ()=>{
  require('electron').shell.openExternal('<LINK_HERE>');
})