In order to provide access to resources that use https, your EZproxy server must be configured with an SSL certificate.
Freegal Music Service requires that the last two lines of config.txt are:
IncludeIP 0.0.0.0-255.255.255.255 SSO -Secret=somesecret -URL=http://www.freegalmusic.com/users/sso?libname=site freegalmusic
where somesecret and site are both provided to you by Freegal.
To access Freegal, you will need to construct a URL similar to:
http:// ezproxy.yourlib.org:2048/sso/freegalmusic
replacing ezproxy.yourlib.org:2048 with your own EZproxy server hostname and port.
The IncludeIP directive forces all users to authenticate. If you omit this directive, browsers accessed by local users will go into a redirect loop.