Map NameID to sub/id in SAML clients

I was wondering if there is any way to map NameID to a user’s id (sub)? I have been successfully using the User Attribute Mapper For NameID to map NameID to user’s emailaddresses but id does not seem to work.

Here is how it works for me using email:

I would like to see it work for id. I am not quite sure where to get the list of attributes from. Based on the description I found keycloak/UserModel.java and tried a few values from there. Some worked, others did not.

Any pointers?