Hi Matthew,

There are a couple of considerations:

1. The validation of fields occurs after the form "submit" event is registered.
2. The field needs to be updated before the form is fully submitted, anyway.

The best way I've found so far is to do some basic validation on my own in the function handler for the Submit button "click" event:

$(document).ready(function() {
  $('form.cf-form').on('submit', function() {
    var noErrors = true;
    // check that all required fields are nonempty
    $('*[required]').each(function() {
      if ($(this).val() == '') noErrors = false;
    });
    // check that any (required) fields with regex patterns match
    $('*[pattern]').each(function() {
      // prepending /^.{0}$|/ to the pattern allows for blank values;
      //   note that any empty required fields of this type will be
      //   handled in the step above
      var patternString = '^.{0}$|' + $(this).attr('pattern');
      var pattern = new RegExp(patternString);
      if (!pattern.test($(this).val())) noErrors = false;
    });
    if (noErrors) $('.fillOnSubmit input').val('Hello');
  });
});

where there is a single-line input with class ".fillOnSubmit" that gets populated with "Hello" if all goes according to plan.

I'm not sure if I missed some other kind of validation, but this might serve your purposes?

Hope this helps!