Passing value to empty form isn't working.

If you're using the code exactly as is, the code is submitting the Marketo form as soon as it's loaded and ready, and most likely when your 'pretty' form is still empty

You want to wrap the Marketo form submission in a click or submit event that triggers the form submission

e.g.

MktoForms2.loadForm("https://app-lon08.marketo.com", "162-XXX-XXX", 10XX); 
MktoForms2.whenReady(function(mktoForm) {
  var myForm = MktoForms2.allForms()[0];
  $('#prettyForm').submit(function(e){
    myForm.addHiddenFields({
      "Email": $("#mwf3723fced2f65").val() ,
      "FirstName": $("#mwfca1d8fb14840").val(), 
      "LastName": $("#mwf2e0f8bf4c5a5").val()
    });
    mktoForm.submit();
  });
});‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍

be mindful if your pretty form is also actually submitting, because it may navigate away from the page before the marketo submission is successful

Please highlight your code using the Advanced Editor's syntax highlighter so it's readable.

And provide as much detail as possible, not just "not working" but a description of your errors/logs and a link to your page.