Getting Hidden Fields to Populate with gform_pre_submission

Here's my function:

add_action('gform_pre_submission_filter_2', 'combinefields');
function combinefields($form) {
  $_POST['input_16'] = $_POST['input_11'];
  return $form;
}

I have not been able get to hidden field 16 populated from checkbox list 11 in form ID2. My main goal is to concatenate two fields into one, so we can send a single field of content to another Add-on to send to Salesforce. I'm stumped why this isn't working with one field. The hidden field has the box checked for "Allow field to be populated dynamically."

Any suggestions?

I don't think that works either. Here's the form and the code I tried.

<script type='text/javascript'>
  jQuery(document).ready(function($)  {
	$("#input_2_12").blur(function()  {
      // sets the value of 12 for what was entered in 11
	  $("#input_2_12").val($("#input_2_11").val());
	});
  });
</script>

Thanks Pierski, but I tried it and still wasn't able to get the hidden field populated, it ends up blank.

I changed the inputs to two single-line visible fields to test, and I couldn't get that to work either. I also tried with two non-hidden fields on a different form, but no luck. No errors in the console.

Any ideas to help narrow the issue down a bit?

I currently have this:

<script type='text/javascript'>
jQuery(document).ready(function( $ ) {
$('#input_1_4').change(function() {
$('#input_1_7').val($(this).val());
});
});
</script>

Set up targeting this form for testing, hoping it would be more obvious than testing with the form with hidden fields.

Let me know if there's anything I can try adjusting. Appreciate your time thus far!

Ah, could that be why the gform_pre_submission_filter wasn't working in the first place? I'll experiment, thanks for the eyeballs!