Actually the needed button was visible the whole time, but instead of being called “Change settings” like the other buttons, it was named something like “Fully host this domain” (which is confusing).
NEW PROBLEM: I added a redirect while debugging this issue and now there does not seem to be a way to turn this redirect off. On the “Manage Domains: Edit” page the direct URL is no longer shown, but it still redirects. When I click “Redirect this domain” (with the “Redirect to URL” field blank), it gives me an error “You need to specify a URL to redirect to!”. No option on this page for simply turning it off.
I could probably just change the .htaccess file manually, but it is not in my FTP.