Contact form - Custom fields are not sending content I've done a custom contact form with 2 more fields, but I can't get values of those 2 custom fields.

I modified contact.module in core to see what are coming from form submit, found my custom fields but they are empty. I just don't know why they are empty.

Value of contact_form in contact.module:

$contact_form = {"contact_form":[{"target_id":"feedback"}],"uuid":[{"value":"5470b680-99db-4d5a-8c00-ae043dce67e1"}],"langcode":[{"value":"pt-br"}],"name":[{"value":"Name"}],"mail":[{"value":"some@mail.com"}],"subject":[{"value":"Any"}],"message":[{"value":"Any message"}],"copy":[{"value":0}],"recipient":[],"field_customa":[],"field_customb":[]}
;


Any clues to make this contact form work?
Drupal 8 will handle custom fields on contact forms 'out-the-box', they will be rendered in the message body by contact module. If custom fields don't appear in message body, someone messed up.

In this particular case, attribute 'name' of 'input' html tags were modified, and $_POST came with a wrong schema. $_POST['field'] instead of $_POST['field'][0]['value'].

Here shows the hint:
"message":[{"value":"Any message"}] - Correct (field[0]['value'])
"field_customa":[] - Wrong (field[])