You need to define a Guest Access service profile, one with No authentication, but does Redirect to the URL you specify.
Then define a WLAN, with this Guest Access profile, and add WPA2-PSK on the WLAN.
Your clients will need the WPA2-PSK in order to associate to your WLAN, and when they open a browser,
they will be redirected to the page you specified in the Guest Access profile.
You must use Guest Access in order to perform the redirection you desire, but you can make it seamless,
and still require that they have the WPA2-PSK to even associate.