CREATE SERVER osm 
FOREIGN DATA WRAPPER nominatim_fdw 
OPTIONS (url 'https://nominatim.openstreetmap.org');
CREATE USER MAPPING FOR foo SERVER osm 
  OPTIONS (proxy_user 'u', proxy_password 'p');
ERROR:  role "foo" does not exist
CREATE USER MAPPING FOR CURRENT_USER SERVER osm 
  OPTIONS (proxy_user 'u', proxy_password 'p');
\deu+
                   List of user mappings
 Server | User name |             FDW options              
--------+-----------+--------------------------------------
 osm    | postgres  | (proxy_user 'u', proxy_password 'p')
(1 row)

ALTER USER MAPPING FOR CURRENT_USER SERVER osm
  OPTIONS (SET proxy_password 'p2');
\deu+
                   List of user mappings
 Server | User name |              FDW options              
--------+-----------+---------------------------------------
 osm    | postgres  | (proxy_user 'u', proxy_password 'p2')
(1 row)

/* unknown user mapping option */
DROP USER MAPPING FOR CURRENT_USER SERVER osm;
CREATE USER MAPPING FOR CURRENT_USER SERVER osm
  OPTIONS (bogus 'x');
ERROR:  invalid nominatim_fdw option 'bogus'
/* clean up */
DROP SERVER osm;