I had the same issue. What I did was to first direct them to my custom HTML page and once they click accept it takes them back to the ZONE director login Portal.
The redirect function doesn't work for me.
So my HTML page has a redirect.
The easiest way for me to get it to work was to put my HTML page on a windows Server in IIS and first send users to that page. The acceptance button then redirects to:
form method=POST action="https://Your-ZoneDirectorURL/user/user_login_auth.jsp";
Hope this makes sense.