short question: how can I set a frontmatter Variable for a specific page via Form ?
(Form page is NOT the page I want to modify)
@hoernerfranz, I wonder what the use-case is for this question…
Anyway, I could think of creating a plugin that responds to the form’s onFormProcessed event, checks the user action defined for the form, finds the page to be updated and does the update.
See the docs on User Actions for some sample code.
@pamtbaau : my usecase is very simple, and I was well aware I could solve this by creating a plugin - but hoped it could be easier 
I have a page which will show different Information based on the value of a boolean Variable in the frontmatter of that page.
of course, that Variable can be set via the Admin Backend, but I wanted to avoid that by just creating a form page which is routable, but not visible in the menu.
The Form simply asks for a Password and should then set the above mentioned Variable via a radio select Field.
The link to the Form Page is then only known to the Operator who does not have to log into the Grav Backend (which would be overkill in this use case)
just for the record: I solved this issue by creating a minimalist plugin which lets the backend user switch a plugin variable via toggle buttons in the Plugin settings page.
the page template then reads that value from the plugin config and displays the desired Text.
so, yes, backend login is now required, but not editing frontmatter (which is not suitable for unexperienced users).