Hello,
here is the solution, it was so quiet stupid simple....
You've to edit the Wireless or Wired LAN with is setup with a WebAuth. Then you can select under the menu topic "Security" the tap "Layer 3" as shown in attachments. Here you select Web Authentication and also select "Over-ride Global Config". Now you can select for all three paged an individual file.
And that it.
If anybody needs some help or an working full customized WebAuth sample, feel free to contact me.
Kind regards
Kai