How to set electron UserAgent

Just use an option object when loading the URL.

function createWindow () {
   win = new BrowserWindow({width: 800, height: 600});
   win.loadURL('http://www.whoishostingthis.com/tools/user-agent/',
     {userAgent: 'Chrome'});

   win.on('closed', () => {
     win = null
   });
}