Add fields to admin content page

How could I add fields to the content page in the Grav backend. I used the first example blueprint of this official doc https://learn.getgrav.org/forms/blueprints/example-page-blueprint and changed "advanced" -> "content", but this doesn´t work.

In this example, this blueprint will append a new field to the section 'override', which is on the advanced tabs. However, the field 'override' is not present on the 'content' tab.
Here is an example that should append to the content section:

title: Gallery
'@extends':
    type: default
    context: blueprints://pages

form:
  fields:
    tabs:
      type: tabs
      active: 1

      fields:
        content:
          fields:
              header.an_example_text_field:
                  type: text
                  label: Add a number 
                  default: 5
                  validate:
                    required: true
                    type: int

Thanks! This works perfectly. Thought override is an defined string for expanding a form with additional fields :|

Yes, that's a bit confusing indeed!