Well, it is just about simply presentate a "Startpage", some sort of Information Portal. It is not about security or something. The goal is simply that the portal page should or better to say is managed by a CMS System. So I need the Portal Page to be hosted external. If this is messy or not is not relevant as this is what is asked for. Anyway, you never could "force" someone to read anything. If the users just want to ignore the information then they will regardless of what I am doing.
There are already some posts about Pre-Authentication Page, but the system here will just work completly without authentication.
The parameter <input name="redirurl" type="hidden" value="$PORTAL_REDIRURL$"> just pass the initaly entert URL (if the CP woks without authentication). So if someone enter the URL www.xyz.com
then the CP just make the redirect to the (local) index.php/html and then, when pressing the Button "Continue", "Accept" or whatever, the CP takes the value of the PORTAL_REDIRURL and the browser will open exactly this URL.
When I redirect the Portal Page to external, then this information is lost, so I have to "hardcode" the Information into the Form. So the URL that is opened after clicking "Continue" will always be the one I entered and the User will have to enter the URL he wanted to reach again. That is uncomfortable for the user and the reason why I am looking for an solution...