Help with checkbox

I have a blueprint that includes a checkbox field:

    header.box:
         type: checkbox
              label: to be tested

But when I make a page in the Admin panel, although the checkbox is displayed, it is always unset when I press Save.

What have I left out?

Did you mean to write “when I press save”? This implies to me you check the checkbox but it’s unset when you go to save the page.

If you mean you want it to default to checked for new pages, use the default field value property. You probably want to set it to ‘checked’ or true, not sure which one works.