Add field to helptext while typing

Kirby 3
1

I want to create a dynamic help text with a link for the panel user to copy.
Basically the same as the page slug help text when you create a new page.
But I can’t figure out how to read the field.

fields:
  subject:
    label: Subject
    type: slug
    help: "{{ page.url }}?subject={{ page.subject}}"