We have setup an external captive portal page for guest access, basically it displays an AUP and forces the user to click on an Accept button. We used this code to make the post back to the instant APs:
<form method="POST" ACTION="http://securelogin.arubanetworks.com/swarm.cgi">
<input type="hidden" name="user" value="guest">
<input type="hidden" name="password" value="guest">
<input type="submit" name="Accept" value="authenticate">
</form>
Now we have two problems, first of all when the user connects, the captive portal page loads just fine but when they click on the Accept button they are redirected to a blank page at http://securelogin.arubanetworks.com/swarm.cgi, but they are allowed onto the network. The second issue is that if the user doesn't click on Accept and instead closes the window, then they are still allowed onto the network.
Here is a screenshot of the blank swarm.cgi page