Notification EXTRA data

sql-notify.html :

Higher-level mechanisms can be built by using tables in the database
to pass additional data (beyond a mere notification name) from notifier
to listener(s).

How ?

root2=> select * from pg_listener ;
relname | listenerpid | notification
---------+-------------+--------------
aaa | 723 | 0
(1 row)

grep -5 "typedef struct pgNotify" /usr/local/include/libpq-fe.h

typedef struct pgNotify
{
char *relname; /* notification condition name */
int be_pid; /* process ID of server process
*/
char *extra; /* notification parameter */
} PGnotify;

I need to insert "extra" data - uid, ...

PGnotify *notify;
notify->extra;

........
Thanks