Configure ExistingCustomDimension

we would like to get unique visitors per page i.e. with Actions.getPageUrls endpoint and save it as Custom Dimension. Would it be possible with configureExistingCustomDimension?

Is there any example what would need to be passed as extractions?

What do you think, @matthieu?

Also when I trigger CustomDimensions.configureExistingCustomDimension as a Super Admin I receive following error:
You can’t access this resource as it requires ‘write’ access for the website id = 1
I also tried with a user with ‘write’ access on this page, but I received the same error message.