These instructions are for Instant 4.0 which allows multiple captive portal profiles. (See page 123 of the Instant 4.0 user guide)
Go to Security > External Captive Portal
Click New
For URL, enter the address to the QuickConnect page.
Then go back and edit the SSID profile and on the security tab, go to the splash page drop-down and select External. Then choose the profile you just created.