https://admin.fedoraproject.org/accounts/openid/id/usernameas URL. Example, if your FAS login is JohnDoe, you would enter https://admin.fedoraproject.org/accounts/openid/id/JohnDoe. If you are not logged in to FAS at this stage, you will be first taken to your FAS login page. On the first login from the external site, you will be taken to admin.fedoraproject.org and asked to approve the OpenID request.
Screenshots are here.
OpenID still does not work for all sites. We are looking for feedback and developers to improve the code. Please leave comments on the Discussion tab. We've also got a list of tested sites you might like to check out in there!
git clone ssh://git.fedorahosted.org/git/fas.git
It's in fas/openid_samadhi.py
It's taken and updated from an upstream project, samadhi
It needed to be modified to work with FAS though.
Many thanks to User:Ricky for his explanations on IRC.