Hello Evan,
Click on the self-registration you want in the admin list, click the Go to login (not Edit) and in the new window copy the url. Use that url instead in your cp profile. If there is no Go to login, it means you havent opened up for Nas login on that particular self-registration.
There is also the option to use the logins created under Customization/web logins. By default they get url like /weblogin.php/2 if you dont want to use the self-registration one.
Hope this answer your question.. If you instead meant where to change the url on the Aruba controller, then you do that under Authentication/ L3 - select and edit the captive portal profile your logon role is using. You should find the login page field there - unless you're running 6.1.4x software in which its bugged so you'll have change it through cli ;)