How to create API accessible user account with Community

Or alternatively you can create a non SSO account in the USERS tab in admin just for API use. Then you should be able to get a session key for that account.

Make sure to make the account an Admin account if you want to use it to retrieve any private information or if you are using FreeMarker you can use restadmin like StanGromer suggested.

Also, if you are only after publicly visible information, LiQl does not require any authentication.