Automatic URL manipulation via header.routes.default

Hello together,
I would like to remove the /folder/ part from the url. (The parent folder)
I wrote
header.routes.default:
type: text
autofocus: true
style: vertical
label: Meine URL
default: "/"
into the system/blueprints/pages/default.yaml and it works fine.

Now, I would like to create or save the page and the header.routes.default should be written automatically into the default . md and especially it should be written default: "/foldername"

So I wouldn't need a extra field on my content area and it wouldn't be written manualy.

Thank you in advance.
Best regards
dci