NodeJs Submit from that's on another page

That is HTML which says it's going to be in a web browser.  You're missing the 'action' attribute of the <form> which tells the browser where to send the information.  NodJs may host the page but it is not used to create the page.  On this page https://www.w3schools.com/html/html_forms.asp , look for The Submit Button and read about the  'action' attribute.

Well, that would be the normal way to do this but I cannot because the form submits to itself.

The form page is on: http://127.0.0.1/ChildPage.html and it's action is action="ChildPage.html"

I'm using electron so I am using Nodejs and general javascript

Please explain what this means

I need to add some code on ParentPage.html that with hit the submit button on ChildPage.html

First - I am assuming that "with hit the submit button" should be "will hit the submit button"
Second - when you talk about parent page and child page - can you explain exactly how this manifests? Are we talking about an iframe'd child page inside a parent?
Are we talking about a child that submits to a parent?

I am assuming you are talking about an iframe - in which case you can call a function in the child page - providing the parent and child originate from the same source / protocol / port

Parent (note script uses jQuery - not shown)

  <div><button class="btn btn-primary">Submit child</button></div>
  <iframe src="t3060-iframe.html"></iframe>
<script>
$(function() {
  $('button').click(function(e) {
    e.preventDefault();
    var target = $('iframe')[0];
    target.contentWindow.submit();
  });
});
</script>

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Child

<form action="reflect.php" id="myform">
  Your Name <input name="name" type="text">
</form>
<script>
function submit()
{
	document.getElementById('myform').submit();
}
</script>

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Working sample

here

Great way of doing this!

Only issue is that I cannot change the code in the child page so:

<script>
function submit()
{
	document.getElementById('myform').submit();
}
</script>

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wouldn't be there.

I would have to just target it saying Form->submit as the form submit button has no id.

Can this be done?

Assuming you have a form with id myform then you could do this

var target = $('iframe')[0]
var doc = target.contentDocument || target.contentWindow.document;
doc.getElementById('myform').submit();

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NOTE: This will only work if the child is on the same URL / PROTOCOL / PORT as the parent - otherwise you can't do the above.

The url is the same and the port too but the form has no id or class.